


Book i L 

dpghtN?- -_ 



COPXRIGHT DEPOSCR 



S 1 JlEjL 



AND ITS HEAT TREATMENT 



BY 
DENISON K. BULLENS 

Consulting Metallurgist 



SECOND EDITION, THOROUGHLY REVISED 
TOTAL ISSUE, FIVE THOUSAND 



NEW YORK 

JOHN WILEY & SONS, Inc. 

London: CHAPMAN & HALL, Limited 
1918 



CA 



v- 



^ A 



<°tft 



Copyright, 1916, 1917 

BY 

.DENISON K. BULLENS 



All rights reserved 



DEC 2! 1917 



10 



^ 



PRESS OF 

ERAUNWORTH & CO. 

ROOK MANUFACTURERS 

BROOKLYN, N. Y. 



>CLA479t>38 



IN MEMORY OF MY FATHER 

Albert $elflfltt |bilens 



if 



Digitized by the Internet Archive 
in 2011 with funding from 
The Library of Congress 



http://www.archive.org/details/steelitsheattreaOObull 



PREFACE TO SECOND EDITION 



In the second edition the scope of this work has been broadened 
to include additional information of a practical nature to illustrate 
further the application of principles in everyday commercial practice, 
and to encourage a consideration of every element in the cycle of 
operations from the initial heating of the steel for forging to the cooling 
in the final heat-treatment process. 

In the section on Heat, additional data are given to illustrate the 
difference between combustion and generation of heat and the 
application of heat to useful work; the difference between the mere 
indication of uniform temperature and uniformly heated product; 
the relation between temperature, time, mass, and surface in the 
determination of uniformly heated product; the influence of furnace 
design and operation on the quality and cost of finished product; 
the weakness of relying upon pyrometer readings without consider- 
irg other equally important factors; and the factors governing the 
selection of furnaces and fuels and the use of both. 

The section on Forging has been materially enlarged into a new 
chapter to illustrate the relation of forging to heat treatment, the 
effect of temperature, time and uniformity of heating upon the 
structure of the steel, together with original photomicrographs illus- 
trating the variation in structure under distinctive conditions. 

The influence of the " Human Element," while not involved in 
a technical consideration of scientific principles, is nevertheless an 
important factor in the practical side of the work, and it is consid- 
ered more in detail for the reason that there has been shown nothing 
so far to prove that it is not essential. It is believed that the prac- 
tice of this art of heat treatment is a specialty or trade in itself, 
requiring skilled men for its proper conduct, and that shop practice 



vi PREFACE 

cannot keep pace with laboratory development until this point is 
recognized and sustained. To encourage favorable consideration of 
this view an effort is made to indicate some of the weaknesses of our 
so-called modern heating practice and to illustrate the relationship 
of furnace and operator to the quality and cost of finished product, 
with suggestions for the improvement of both. 

The chapter on Annealing has been enlarged with considerable 
data of interest to the practical man, illustrating the effect of tem- 
perature, time, and saturation in both heating and cooling upon the 
grain size and uniformity of structure, and thus more fully empha- 
sizing in a practical manner the basic principles of the effect of heat 
upon steel. Also there is shown a series of unusual photomicro- 
graphs which illustrate most graphically the theory of " diffusion 
and equalization " advanced in the previous edition. 

The number of photomicrographs and other illustrations has 
been materially increased to supplement the written matter, and the 
whole is based substantially upon observations made under actual 
working conditions. 

D. K. BULLENS. 
Philadelphia, 

July, 1917 



PREFACE TO FIRST EDITION 



Modern Heat Treatment should be considered as an art or trade, 
since it certainly requires knowledge, skill and judgment for its 
proper performance. These, in turn, necessitate at least some knowl- 
edge of heat, of steel, and of the effect of heat upon steel. And all 
three factors are linked together by the " human element." The 
author has therefore endeavored to bring together the theoretical and 
practical sides of the general subject of steel and its heat treatment 
in such a manner as will, he hopes, be understandable by that 
" human element." 

The theoretical or scientific side of heat treatment is not affected 
by a consideration of many elements which enter into the art or 
practical application of metallurgical principles, but unless per- 
formance in the shop compares favorably with the standard set in 
the laboratory, the value of the work is lessened. The technical 
principles have been dealt with heretofore more thoroughly than 
the application of these principles, and it is the purpose of this 
book more completely to consider this practical aspect in the hope 
that a better understanding of the factors governing commercial 
practice mry suggest means of improvement in shop methods. 

It has been the author's attempt to make the chapters dealing 
with the heating problem illustrate the necessity for considering all 
of the elements entering into the production of a uniformly heated 
product; of heat application rather than details of construction; 
of the importance of the human element and the relation it bears 
to the ultimate results; of encouraging a broad and common-sense 
view of the principles governing this important operation and 
which generally have been overlooked; and finally, of viewing the 



viii PREFACE 

heating problem as an engineering proposition, adapting each fuel 
to proper furnace design and operation to meet the requirements 
of the problem in hand, and by so doing aim for the adoption of the 
standard heating unit in terms of finished product — " the cost of a 
unit of quantity of given quality." 

He has attempted to make as practical as possible those chap- 
ters relating to steel and the effect of heat upon steel. Theories 
have been advanced only so far as has been thought necessary for 
a clear understanding of principles. Wherever possible, illustra- 
tions in the form of photomicrographs and charts have been given. 
The data given under the various types of heat-treated steels have 
been checked as far as possible and every effort has been made to be 
correct. 

To the many friends who have aided him in the preparation of 
this book the author would express his sincere appreciation. Effort 
has been made to give due credit for cuts and data at the proper 
place, and for such as may not have been made, acknowledgment is 
hereby rendered. 

Denison K. Btjllens. 
Philadelphia, 

October 1, 1915. 



NOTE TO SECOND IMPRESSION WITH ADDITIONS 



To comply with requests of readers of the First Edition of 
this book for concise information upon the treatment of modern 
high-speed steels, it has been deemed advisable to add an Appendix 
on this subject. 

D. K. B. 



CONTENTS 



HAPTER PAGE 

I. The Testing of Steel 1 

II. Heat Generation 16 

III. Heat Application 32 

IV. The Human Element 73 

V. Forging 82 

VI. The Structure of Steel 93 

VII. Annealing 116 

VIII. Hardening 159 

IX. Tempering and Toughening 191 

X. Case Carburizing .' 210 

XL Case Hardening: Thermal Treatment 251 

XII. Carbon Steels 269 

XIII. Nickel Steels 294 

XIV. Chromium Steels 332 

XV. Chromium Nickel Steels 349 

XVI. Vanadium Steels 378 

XVII. Manganese, Silicon, Tungsten, and Molybnedum Steels. . . . 387 

XVIII. High-speed Steels 403 

XIX. Tool Steel and Tools 411 

XX. Miscellaneous Treatments 440 

XXI. Pyrometers and Critical Range Determinations 460 

Index 475 

ix 



STEEL AND ITS HEAT TREATMENT 



CHAPTER I 

THE TESTING OF STEEL 

Growth of Heat Treatment.— Probably no one division in the 
metallurgy of steel has taken such wonderful strides in recent years 
as has the art of heat treatment. Twenty years ago the scientific 
knowledge and technical application of heat treatment were but 
very limited. Such as it was, it usually consisted in " heating to a 
red heat " for annealing, or perhaps the instructions called for 
" harden at a bright red and temper to a straw color." Then it 
was an art guarded with much secrecy and confined for the most 
part to makers of tools and a certain few specialties. 

Practically all alloy steels require treatment of one sort or another. 
In the " natural " state very few steels present their full value, so 
that heat treatment is not only advisable but often mandatory. 

Necessity for Heat Treatment.— Take for example the steels 
used in the automobile industry. The frame requires resistance to 
vibratory stresses occasioned by rough roads, as well as strength and 
toughness. Rear axles must have great tortional resistance; front 
axles " must withstand vibrations. The steering parts must be 
strong, tough and without brittleness; the springs must neither sag 
nor break. Crank-shafts must be able to resist impact, besides being 
stiff. Gears are subject to wear and must be capable of withstanding 
this action if a smoothly running transmission is to be had. And 
so each separate part might be named, all having a more or less 
severe duty to perform and requiring steels possessing various 
degrees of strength, toughness, resilience, endurance, shock-resisting 
and wearing qualities. 

Testing. — These various combinations of static and dynamic 
strength are obtained by adjusting and correlating both the chemical 



2 STEEL AND ITS HEAT TREATMENT 

composition and heat treatment of the steel. Certain chemical 
components intensify the static properties of the material; others 
may affect the dynamic qualities. Thus by coupling with a steel of 
suitable chemical combination the proper heat treatment, there 
arises a product with physical properties most adapted for the work 
in hand. Similarly, having once produced a suitable article, it then 
remains to duplicate it. To this end all rational heat treatment 
must be aligned and standardization of results be obtained. In 
order to accomplish these specific requirements, the influence of 
definite chemical composition and definite treatment must be known, 
as will be described in later chapters. The guide to this work is 
frequent and constant testing, and a definite knowledge of the vari- 
ous components should be possessed by every heat-treatment man. 
Thus we may say that the purpose of practical testing is (1) to sup- 
ply information as to suitable material and its qualities for different 
purposes, both for the manufacturer of the material and for the 
designer or user, and (2) to test the specified uniform quality of the 
material. 

Stresses and Strains. — Testing resolves itself into a determination 
of the strength of the material, which in turn is measured by the 
application of a force and its resultant effect. The force put upon 
a body is termed the stress, and the deformation resulting from that 
force is the strain. Upon the method of applying that force depends 
the nature of the test. Thus we may conveniently classify such 
stresses under the following headings: 

A. Steady or constant loads- — static stresses; 

B. Repeated static stresses and accelerated stresses — fatigue 

stresses; 

C. Suddenly applied loads — impact stresses; 

D. Repeated impact or vibratory stresses — dynamic stresses; 

E. Miscellaneous tests such as resistance to penetration, wear, 

etc. 

Tensile Strengths — The most common test for static strength, 
that is, the strength of the steel under constant load and without 
shock or vibration, is the tensile test. Thus we may call the tensile 
strength the absolute strength of the metal under tension, i.e., the 
force actually required to pull the metal asunder. A standard test 
piece is gripped between the upper and lower jaws of a testing machine 
and the total resistance to rupture is measured. Knowing the area 
of the cross-section of the test piece and the load required to break 



THE TESTING OF STEEL 3 

it, the strength per square unit may then be calculated. The tensile 
strength is usually given in pounds per square inch (American) , 
tons per square inch (British), or kilograms per square millimeter 
(metric system; 1 kg. per mm. 2 =1422.32 lbs. per square inch). The 
accuracy of the tensile test is dependent not only upon the conditions 
under which the test is made, such as the rate of pulling, alignment 
of test piece in the machine, etc., and which are more or less influenced 
by the human element, but also upon the metal itself. The higher 
the tensile strength and brittleness of the steel, the greater the 
possibility of error; differences of several thousand pounds per 
square inch are often encountered in the same piece of high-tensile, 
heat-treated steel, even in the absence of brittleness. 

Test pieces are generally taken half way between the center and 
the outside of the piece, and longitudinally or " with the grain." 
Occasionally it is necessary to take tests transversely or " across the 
grain"; in this case the results will be lower than in the longitudinal 
test, the exact amount depending upon the composition and treat- 
ment of the steel. 

The static strength increases in direct proportion to the carbon 
content of the steel. For the ordinary basic and acid open-hearth 
steels, without heat treatment, Campbell gives the following formulae 
by which the tensile strength of such steels may be roughly deter- 
mined. These results apply for steel " in the natural." 

Acid open-hearth steel : 

Tensile strength = 40,000 + 1000C + 1000P+zMn. 
Basic open hearth steel: 

Tensile strength = 41, 500+ 770C + 1000P+yMn. 

In these formulae, C equals each one point (0.01 per cent.) of 
carbon as determined by combustion, P equals each 0.01 per cent, 
of phosphorus, Mn equals each 0.01 per cent, of manganese, and x 
and y are given in the table on page 4. 

Elastic Limit (Tension). — The term " elastic limit " has probably 
been more ill-used than any other common technical testing name, 
with the possible exception of " hardness." Among its many 
definitions the two which stand out pre-eminently are (1) the least 
stress at which the material retains a permanent deformation or 
" set " after the removal of the stress; and (2) the least stress under 



STEEL AND ITS HEAT TREATMENT 



Percentage of Carbon. 


On Acid Steel. 

X 

Lbs. per Sq. In. 


On Basic Steel. 
V 

Lbs. per Sq. In. 


0.05 ; 




not 

130 
150 
170 
190 
210 
230 
250 


0.10 

0.15 


80* 
120 
160 
200 
240 
280 
320 
360 
400 
440 
480 


0.20 


0.25 


0.30 


0.35 


0.40 


0.45 


0.50 


0.55 


0.60 





* Beginning only with 0.4 per cent, manganese, 
t Beginning only with 0.3 per cent, manganese. 

which ductile material exhibits a marked yielding — sometimes 
denoted as the " yield point." 

The determination of the true elastic limit should always be 
taken from a curve plotted, using an extensometer, from a series 
of careful observations, as otherwise sets caused by non-homo- 
geneity and initial stress might be obtained which do not repre- 
sent the plasticity of the material. This method of determining 
the elastic limit is but little used commercially, as the amount of 
labor involved is too great. 

The yield point, or commercial elastic limit, is obtained by 
noting the stress at which the test piece first begins to " give " or 
elongate. This may be obtained by means of two prick-punch marks 
and observing the first signs of elongating by means of dividers held 
on these points; or by noting the drop of the weighing beam or halt 
in the load indicator (" jockey ") ; or by means of the general appear- 
ance of the test piece. 

In its practical application the elastic limit may be called the 
working strength of the material, for in most cases the steel or 
machine part becomes useless when strained beyond its elastic 
limit. This is particularly true of automobile construction, in 
which the value of a car is dependent upon the correct adjustment 
and alignment of its several working parts, such as in transmissions 
and transmission suspensions. All tests given in this book, unless 
otherwise noted, refer to the commercial elastic limit or yield point. 

The relation existing between the elastic limit and the tensile 



THE TESTING OF STEEL 5 

strength is too broad a subject for discussion here, as the varying 
chemical compositions and heat treatments exert such a tremen- 
dous influence; a study of the results given in following chapters 
will show a proportionality of 40 per cent, and upward. 

Elongation. — The elongation is measured in per cent, of the 
original test section and is commonly the amount of stretch which 
will occur in the material when pulled apart by tension. It is 
usually measured in relation to an initial distance of 2 or 8 in., 
or 100 mm. when the metric system is used, but other specifica- 
tions as used in Europe give a definite relation of original gauge- 
length to the thickness or diameter of the specimen. 

Reduction (or Contraction) of Area. — The reduction of area 
refers to the area at the point of rupture, usually reported in per 
cent, reduction of the original area — that is, the original area of the 
test piece minus the area of the smallest cross-section after frac- 
ture; this divided by the original area is the percentage reduction of 
area. The reduction of area should be regarded as a measure of 
the saturation of the steel at the proper temperature. 

Ductility. — The percentage elongation and percentage reduction 
of area are a measure of the " ductility " of the material, usually 
varying inversely with the tensile strength. The true measure of the 
ductility of the steel cannot be taken alone from either the elonga- 
tion or reduction of area, as the results obtained in either case will 
depend in a large measure upon the size of the test piece, the method 
of testing, etc. Many engineers regard the reduction of area as the 
more reliable; this is offset by the fact that many steel specifications 
make no mention of the reduction of area, but particularly specify 
the percentage elongation. Ductility may also be defined as the 
amount of distortion of the material before final rupture. 

Compressive Strength. — The compressive strength of material 
is its resistance to crushing. The test is generally carried out upon 
a small cylinder or 1-in. cube of the metal, using the same machine 
as for the tensile test. Care must be used to see that the line of 
strain passes exactly through the axis of the specimen, and that the 
plates above and below the piece have a greater resistance to pene- 
tration than the metal to be tested. The application of the term 
elastic limit is similar to that in the tensile test. 

Torsional Strength. — As its name implies, the torsion test is used 
to determine the resistance to twisting. This test is very largely 
used to-day for automobile steel and is measured in inch-pounds 
with the amount of distortion given in degrees. The elastic limit is 



6 STEEL AND ITS HEAT TREATMENT 

obtained as in a tension test, using either a tropometer or an auto- 
graphic attachment. The usual comparison is by calculating the 
shearing stress in pounds per square inch. 

Endurance. — The computation and understanding of such static 
stresses as have been previously outlined are comparatively simple. 
The requirement to be fulfilled in designing is that the working stress 
shall not exceed the elastic limit of the material, whether it be 
in tension, compression or torsion. Numerous every-day failures, 
however, which cannot be accounted for by the limited information 
given by such tests, have forced investigators to probe more deeply 
into the complicated kinematic forces which seem to have such a 
great influence upon the " life " of the metal. It is now a well- 
known fact that, if a stress is applied a great number of times, i.e., 
repeated, each application being made before the material has had 
time to recover from the preceding stress, the material will event- 
ually break even though the stress is below the elastic limit of the 
material. These repeated stresses upon steel cause a gradual dis- 
turbance of the structure and its component particles, which greatly 
weakens the material, and is called fatigue. The resistance to 
fatigue and its numerical test value may be termed the endurance 
of the steel. The stresses embodied under the heading of fatigue 
may be broadly classed as repeated static stresses and acceleration 
stresses ranging from zero to maximum or from a negative maximum 
to a positive maximum — alternating — stresses. 

Fatigue Stresses. — These stresses are produced in a machine 
part by an external force or forces of varying strength and direction 
acting upon the part. When the force is produced by a continu- 
ously varying acceleration or retardation of masses taking part in 
the movement of the machine part, they may be conveniently termed 
acceleration stresses. 1 Typical stresses of this category are the 
revolving shaft stress on a loaded wheel or machine axle, the piston 
pressure and the acceleration pressure of the movable parts in the 
piston rod and crank-shaft of high-speed steam and oil engines. 
These perpetual stresses or so-called fatigue stresses are the essential 
ones in the movable parts of most high-speed machines, and a knowl- 
edge of the capacity of the material to resist them should serve as 
a basis for the selection of the material and design. 

Rotary Bending. — Such static endurance tests may be carried 
out in a machine of the rotary bending type, such as the Wohler 
or the White-Souther machines. From a study of a large number of 

1 J. O. Roos af Hjelmsaeter, Int. Assoc. Test. Mat., 1912, Vol. II, No. 9. 



THE TESTING OF STEEL 7 

experiments made on a rotary bending machine of the Wohler 
design, Foos concludes that such endurance tests are not suitable as 
specification tests, but are of great value in the selection of material 
and the heat treatment for various purposes. 

On the other hand, the real value of the rotary bending test as a 
criterion of the brittleness-fatigue endurance has been of late greatly 
questioned. That the results usually obtained are largely indicative 
of the elastic limit alone is probably more in accord with our present- 
day knowledge. The results from a series of tests conducted by 
Foos upon a Wohler type rotary bending machine with steels of 
0.11, 0.40 and 0.65 per cent, carbon, given in the following table, 
would tend to support the latter theory, as one would naturally 
expect from past experience that the 0.40 per cent, carbon steel would 
have a greater fatigue-resisting strength than the 0.65 carbon steel. 

ROTARY BENDING TESTS, WOHLER MACHINE 







Chemical. 


Static Propert 


es. 


Endurance 
Limit. 




















Fiber 






















Stress 








oj 


oi 






Tensile 


Elastic 


Elonga- 


in Kg. 


02 




s 


a 
o 

M 

OS 

O 


a 

03 
ox 

el 

03 


t-t 

o 

-a 
p. 

m 

o 
-a 
Ph 


u 
3 
ft 

n 

3 
m 


o 
o 

m 


Strength 
Lbs. per 
Sq. In. 


Limit 
Lbs. per 
Sq. In. 


Per Cent 

in 
3.94 Ins. 


giving 
Fracture 

after 
1 Million 
Revolu- 
tions. 


R 


A 


0.11 


0.33 


0.019 


0.013 


0.01 


49,770 


32,990 


34.1 


16 


Si 


A 


0.40 


0.51 


0.027 


0.011 


0.20 


82,760 


50,770 


23.8 


22 


s 2 


O.T. 


0.40 


0.51 


0.027 


0.011 


0.20 


109,780 


70,820 


15.6 


28 


Ti 


A 


0.65 


0.49 


0.023 0.007 


0.20 


116,040 


50,900 


14.3 


25 


T 2 


O.T. 


0.65 


0.49 


0.023 0.007 


0.20 


151,020 


94,560 


11.0 


38 



Treatment. " A," heated at 1560° F. for 30 minutes, and air-cooled. 

" O.T.," heated at 1560° F. for 30 minutes, quenched in mineral oil, and 
re-heated to 1025° F. 



Suddenly Applied Loads. — Machine parts at one time or another 
may be exposed to abnormal working loads such as may result from 
a single accidental blow or a sudden retardation of masses in motion, 
and which in any case cannot be supposed to be frequently repeated. 
Such abnormal stresses are therefore in the nature of suddenly applied 
loads or impact stresses, and constitute a different group from those 
previously discussed under static stresses. It is evident that such 
stresses demand that the material be able to sustain a great work of 
deformation for a single or a few impacts without rupture — that is, 



8 STEEL AND ITS HEAT TREATMENT 

the machine part shall sustain as little damage or deformation as 
possible. In general the " ductility " (elongation and reduction of 
area) has for a long time been used as a measure of the work of rup- 
ture. But, although such tests are of comparative value, they do 
not measure either the ductility under impact or the strength or 
resistance under impact. 

Drop Test. — The drop test as commercially applied may be 
described as an aggravated bend test on a large scale. It is a relative 
or qualitative test only, usually made on a full-size forging, to deter- 
mine roughly the homogeneity of the metal and its ductility under 
shock. We have arbitrarily separated this test from the " impact " 
tests, reserving the latter %s applied to the specific measurement of 
the impact strength upon a more or less standardized test bar. 
The most common application of the drop test is that of locomotive 
axles, in which it is required that the axle shall stand a specified 
number of blows at a given height without rupture and without 
exceeding, as a result of the first blow, a certain deflection. 

Impact Strength. — The impact test is used to determine the 
ability of the metal to withstand a suddenly applied load in the 
nature of an impact or shock, thus detecting brittleness or lack of 
toughness. This function is called resilience by the foreign technical 
world, referring to fragility or the converse of brittleness, and is 
stated in terms of the specific work of rupture under impact. It 
should not be confused with the English word "resilience," which 
is interpreted in this country as " springiness." This fragility is 
not defined by the tensile test, although an experienced steel man, 
from an examination of the size and aspect of the grain and other 
conditions of the fracture of the test piece, can usually express an 
opinion as to the fragility, but he cannot assess any definite value. 
Although the drop test specifies the fragility in a qualitative manner, 
it does not measure the actual resistance to rupture, and is therefore 
but an imperfect test. In order to overcome such objections and 
to arrive at a definite value, machines have been devised which 
break a special notched bar by a blow — the force required to rupture 
the metal being measured in kilogram-meters or foot-pounds. 
Notched test bars are used in order to localize the deformations. 
The blow must be delivered with sufficient velocity to bring out the 
desired brittleness functions. This blow or impact may be obtained 
by a falling weight (the Fremont machine), by a falling pendulum 
(the Charpy principle), or by a revolving fly-wheel bearing a releas- 
able knife (the Guillery machine), these three representing the most 



THE TESTING OF STEEL 9 

common types of impact machines in use abroad, as well as the 
Olsen pendulum type (using a test specimen in the form of a canti- 
lever) in this country. 

Impact Tests. — Fremont x recommends as the ideal conditions 
to be realized in the application of the impact test: (1) A minimum 
drop of four meters, or proportional to the impact speed; (2) the 
weight of the anvil block to be equal to at least forty times the 
weight of the tup; (3) sufficient ease and rapidity of adjustment of 
the machine. 

On account of the many factors entering into the problem and 
the numerous designs of impact testing machines, the majority of 
the testing associations have abstained from prescribing any special 
type of apparatus for performing the test. The Copenhagen Con- 
gress (1909) of the International Society for Testing Materials has, 
however, recommended the use of a standard notched test bar of 
30X30X160 mm., or a smaller bar of 10X10 mm. cross-section 
when the larger size is not available. On the contrary, there are 
many who believe that a smaller, rectangular test bar reveals more 
clearly the local defects which form the nucleus of future cracks, etc. 

Use of Impact Tests. — The fragility test has a double purpose — 
to point out steel which is defective, either inherently or by incorrect 
heat treatment; and to act as a valuable aid for the adjustment of 
a proper heat treatment. It is evident that steels burdened with 
sulphur and phosphorus, or rotten with piping and segregation, will 
always remain brittle whatever one may do. But ordinary, sound 
stock, properly treated, is nearly always non-brittle. The degree 
of brittleness will of course vary according to the composition, 
treatment and use of the different steels. The effect of heat treat- 
ment upon the impact strength is very great, so that due care should 
be used in so adjusting the chemical composition and treatment of 
the steel as to give the best combination for the work in hand. 
The impact test, in conjunction with other tests, gives a quick 
method of determining a quality the importance of which is yearly 
becoming more prominent scientifically; commercially, however, 
the impact tests are so unreliable, or vary so greatly, that they can 
hardly be used with any degree of accuracy. 

Fatigue Impacts. — An impact or shock has a considerably greater 
effect than a stress slowly applied, and if repeated a sufficient num- 
ber of times will eventually result in the rupture of the specimen. 
When such frequently repeated stresses are comparatively small — 

iCh. Fremont, Proc. Int. Assoc. Test. Mat., Vol. II, No. 9, 1912. 



10 STEEL AND ITS HEAT TREATMENT 

that is, are well below the elastic limit of the material — they may be 
termed fatigue impacts. Their measurement, as determined by the 
energy or amount of work they represent, is a principal component 
of the dynamic strength of the material. Apparatus for thus test- 
ing the material is developed upon the principle of alternating 
impacts. As practical examples of such stresses in commercial 
application there may be mentioned the stresses to which the axles 
of locomotives or railway cars are subjected every time the wheel 
passes over a rail joint, or the impact stresses sustained by different 
parts of a motor car when passing over bad roads, or in changing to 
different speeds, etc. 

Alternating Impact Machines. — Various machines for establishing 
comparative numerical values for this dynamic strength have been 
designed, in the endeavor to produce an alternating flexure and at 
the same time deliver a blow or impact. This has been accomplished 
by applying blows to the upper part of a test piece with the aid of 
two pendulum balls which, are made to fall alternately from opposite 
directions from a certain height against the test piece. Other 
machines have been patterned along the lines of the Upton-Lewis 
machine, keeping the fiber stress well within the elastic limit. 

Alternating Impact Test Results. — The study of a great number of 
alternating impact tests of a comparative nature, the views of other 
engineers, and the study of steel parts broken in service, would lead 
the author to the opinion that the dynamic strength (using the term 
in its broadest meaning) of straight carbon steels reaches a maxi- 
mum at 0.25 to 0.35 per cent, carbon, with perhaps even narrower 
limits of 0.25 to 0.30 per cent, carbon. Further, the maximum 
endurance is obtained when the steels have been properly hardened 
from a temperature slightly over the upper critical range and tough- 
ened at a temperature of 1200° to 1250° F. The maximum com- 
bination of static working strength, ductility, resistance to shock 
and vibration, and endurance probably is obtained in straight car- 
bon steels with 0.35 to 0.45 per cent, carbon when subjected to the 
above treatment. 

Relation of Various Tests. — There does not seem to be any simple 
relation between the elastic limit at steady tensile stress and the 
limit of endurance in the rotary bending test, although some engineers 
consider it as a " reflection of the elastic limit." The rotary bending 
is undoubtedly less than the former, and according to the investiga- 
tions of some engineers, the limit of endurance (rotary bending) will 
generally amount to about 50 to 80 per cent, of the elastic limit. 



THE TESTING OF STEEL 11 

According to experiments made by Foos on straight carbon steels, 
the limit of endurance for rotary bending and alternating impact 
within the elastic limit agree fairly closely. 

A high value in the work of rupture in the impact test may be 
considered to give comparative security in the case of occasional 
abnormal over-loads. 

Thus, the requirements for a high-quality steel for machine parts 
are a high limit of endurance for the normal stresses and a high figure 
of rupture for the abnormal stresses. As a rule, these qualities are 
opposed to each other in ordinary materials, and it must rest upon 
experience as to which to give the preference; in parts which suffer 
through vibration and other fatigue stresses, it will probably be 
wiser to give preference to the endurance properties. It must be 
remembered that heat treatment and the various alloys may 
entirely change the different properties. 

Hardness. — " Hardness x may be defined as the property of 
resisting penetration, and conversely, a hard body is one which, 
under suitable conditions, readily penetrates a softer material. 
There are, however, in metals various kinds or manifestations of 
hardness according to the form of stress to which the metal may be 
subjected. These include tensile hardness, cutting hardness, abra- 
sion hardness, and elastic hardness; doubtless other varieties could 
also be recognized when the experimental conditions are modified 
so as to bring into operation properties of the material in addition 
to that of simple, or what may be conveniently called mineralogical 
hardness. This has been defined by Dana as ' the resistance offered 
by a smooth surface to abrasion.' The usual quantitative tests for 
hardness are static in character, but the conditions are profoundly 
modified when the penetrating body is moving with greater or less 
velocity. The resistance to the action of running water; to the 
effect of a sandblast; or to the result of the pounding of a heavy 
locomotive on a steel rail, afford examples of what might perhaps for 
purposes of distinction be called dynamic hardness, which is a 
branch of the subject which has received little quantitative 
examination." 

Brinell Hardness. — The Brinell test consists in the pressing of a 
hardened steel ball into the surface of the object under test by 
means of a fixed load. The dimensions of the impression thus ob- 
tained form the basis for calculating the hardness. If the number 
of kilograms making up the load is divided by the spherical surface 
1 Thomas Turner, Inst. Journ., May, 1909. 



12 STEEL AND ITS HEAT TREATMENT 

of the impression, expressed in square millimeters, a number is 
obtained, expressing the pressure exerted per square millimeter of 
ball impression. This number is now accepted as a measure of 
hardness, and it is hence called the Brinell hardness number. It is 
generally sufficient to utilize the diameter of the ball impression 
itself as a measure of the hardness. In order to make tests ex3- 
cuted at different works directly comparable, a standard ball of 10 
mm. and a load of 3000 kilograms are used. 

Brinell Transference Number. — It is a well-established fact that 
the Brinell hardness numbers follow very closely the tensile strength 
of the same types of steel, whether the steel be " in the natural," 
or whether it has been subjected to some heat-treatment process. 
For this reason it is particularly applicable to the rapid testing of 
steel from which it would be impracticable to take regular tensile 
tests. A few comparisons between the actual tensile strength as 
obtained by pulling tests and the hardness number obtained from 
the test pieces will serve - as a basis for future calculations. By ob- 
taining such a " transference number " the probable tensile strength 
of the steel in question may be easily computed 1 by multiplying the 
hardness number by the " transference " number. This transference 
number will vary with the chemical composition of the steel, and 
to a small extent with the manner of testing (whether with or across 
the grain) and between tempered and annealed steels. On the whole, 
however, the test is comparatively accurate for steels purchased 
or made under the same general chemical specification. For straight 
carbon steels this transference number may be said to be about 
500 to 520. The Brinell method has a great advantage over the 
scleroscope in that it does not require an extremely smooth or 
polished surface for the test; the removal of scale by filing is prac- 
tically the only requirement. 

Many companies are standardizing their heat treatment product 
by taking the hardness of each piece treated, thus ensuring a close 
range of the desired tensile properties. On account of the influence 
of the size of the original section upon the physical results as obtained 
by the pull-test, it is much easier to determine the 1 transference 
number for the specific grade of steel being treated, and then vary 
the toughening temperature so as to give the desired Brinell hardness 
number. This method is fairly accurate and the Brinell number will 

1 For carbon steels Abbott eives the following equation as representing the 
relation between tensile strength and Brinell hardness: Tensile strength = 
1000 [0.73 Brinell -28] lbs. per sq. in. 



THE TESTING OF STEEL 



13 



give a close approximation of the true tensile strength regardless of 
whether the treated bar is 1 in. or 5 in. thick, provided that the 
steel has been thoroughly saturated at the proper temperature. The 
Brinell method is simple and commercially efficient, with the excep- 
tion of either very thin or highly tempered material. 

For reference convenience, the relation between the diameter of the 
impression and the hardness number is given in the following table: 



BRINNELL'S HARDNESS-NUMBERS 
Diameter of Steel Ball = 10 mm. 



Diameter 
of Ball 
Impression 
mm. 


Hardness 
Number for 
a Load of 
3000 Kgr. 


Diameter 
of Ball 
Impression 
mm. 


Hardness 
Number for 
a Load of 
3000 Kgr. 


Diameter 
of Ball 
Impression 
mm. 


Hardness 
Number for 
a Load of 
3000 Kgr. 


Diameter 
of Ball 
Impression 
mm. 


Hardness 
Number for 
a Load of 
3000 Kgr. 


Diameter 
of Ball 
Impression 
mm. 


Hardness 
Number for 
a Load of 
3000 Kgr. 


2 


946 


3 


418 


4 


228 


5 


143 


6 


95 


2.05 


898 


3.05 


402 


4.05 


223 


5.05 


140 


6.05 


94 


2.10 


857 


3.10 


387 


4.10 


217 


5.10 


137 


6.10 


92 


2.15 


817 


3.15 


375 


4.15 


212 


5.15 


134 


6.15 


90 


2.20 


782 


3.20 


364 


4.20 


207 


5.20 


131 


6.20 


89 


2.25 


744 


3.25 


351 


4.25 


202 


5.25 


128 


6.25 


87 


2.30 


713 


3.30 


340 


4.30 


196 


5.30 


126 


6.30 


86 


2.35 


683 


3.35 


332 


4.35 


192 


5.35 


124 


6.35 


84 


2.40 


652 


3.40 


321 


4.40 


187 


5.40 


121 


6.40 


82 


2.45 


627 


3.45 


311 


4.45 


183 


5.45 


118 


6.45 


81 


2.50 


600 


3.50 


302 


4.50 


179 


5.50 


116 


6.50 


80 


2.55 


578 


3.55 


293 


4.55 


174 


5.55 


114 


6.55 


79 


2.60 


555 


3.60 


286 


4.60 


170 


5.60 


112 


6.60 


77 


2.65 


532 


3.65 


277 


4.65 


166 


5.65 


109 


6.65 


76 


2.70 


512 


3.70 


269 


4.70 


163 


5.70 


107 


6.70 


74 


2.75 


495 


3.75 


262 


4.75 


159 


5.75 


105 


6.75 


73 


2.80 


477 


3.80 


255 


4.80 


156 


5.80 


103 


6.80 


71.5 


2.85 


460 


3.85 


248 


4.85 


153 


5.85 


101 


6.85 


70 


2.90 


444 


3.90 


241 


4.90 


149 


5.90 


99 


6.90 


69 


2.95 


430 


3.95 


235 


4.95 


146 


5.95 


97 


6.95 


68 



Shore Scleroscope. — The principle of the Shore scleroscope hard- 
ness test is based upon the height of rebound of a diamond-faced ham- 
mer when dropped from a standard height upon the surface of the 
material to be tested. One of the great disadvantages of the sclero- 
scope is that it requires a highly polished surface in order to obtain 
anywhere near accurate and comparative results. The scleroscope 
readings are probably more indicative of the elastic limit than of the 
tensile strength. The following experiments by Swinden, made with 
the scleroscope, illustrating the results to be obtained with different 



14 



STEEL AND ITS HEAT TREATMENT 



methods of polishing, may afford some explanation of the variations 
often characteristic of this instrument : 

Specimen rough filed readings, 25 to 30 

smooth filed readings, 27 to 32 

rubbed with emery cloth No. 1 readings, 32 
rubbed with emery paper No. 00 readings, 32 
polished with diamantine readings, 33 

Ballistic Test. — The ballistic test is distinct from the static hard- 
ness tests above described in that it is a measure of the dynamic 
hardness by resistance to penetration under violent impact. With 
protective-deck and bullet-proof steel the Brinell hardness is only a 
measure of the tensile strength and does not give a comprehensive 
idea of the ballistic qualities of the plate or sheet. Similarly, tests 
made by the Italian Government show that none of the tests men- 
tioned in this chapter, either static or dynamic, nor their ensemble, 
gives a sufficient indication of the resistance which such plates will 
oppose in firing tests. 

Wear. — Wear, or the hardness of material as indicated by its 
resistance to abrasive action, has demanded considerable attention 
of late on account of the increased development of high-power 
machines and engines. The increased weight put upon locomotive 
axles and rails, the higher speeding of rotating parts, and a similar 
tendency to wear in other machine parts have all necessitated 
further study of this important property of steel. 

For resistance to abrasion, Robin i has arrived at the following 
values, these being obtained upon annealed steel with carbon con- 
tents as given, manganese — 0.25 per cent, to 0.30 per cent.; phos- 
phorus — 0.015 per cent, to 0.40 per cent.; silicon — about 0.20 per 

cent. : 

WEAR BY ABRASION. ANNEALED STEEL 



Carbon 
Content 
Per Cent. 


Abrasive 
Figure. 


Carbon 
Content 
Per Cent. 


Abrasive 
Figure. 


0.07 
0.12 
0.25 
0.38 
0.60 


295 
293 
312 
350 
312 


0.65 
0.69 
0.83 
1.00 
1.03 


308 
280 
258 
252 
252 



1 J. Robin, Inst. Journ., II, 1910. 



THE TESTING OF STEEL 15 

This would tend to show that the wear is not proportional to the 
carbon content in annealed carbon steels, and that the maximum 
wear might be expected with steels of approximately 0.40 per cent, 
carbon. He further concludes from other experiments that the 
abrasive wear increases with the percentage of phosphorus and 
diminishes with the amount of manganese and silicon. 



CHAPTER II 
HEAT GENERATION 

Distinctive Conditions. — In industrial heating, and particularly 
the sequence of operations applied to the heat treatment of steel, 
it is but hard common sense to state that there is no general solution 
applicable to the heating element or furnace. The application of 
heat to these various operations, with the accompanying design of 
furnace equipment, is an engineering problem, and it must be con- 
sidered as such, and in the broadest manner, if the greatest efficiency 
is to be obtained. No single type of furnace, fuel nor " system " 
of burning fuel can be applied as a " cure-all." Each case must be 
dealt with on its merits and the furnace and the fuel and their appli- 
cation to the work in hand must, in the final analysis, be based upon 
the results sought for, measured commercially. Consequently, as no 
two problems are exactly alike, it necessarily follows that the furnace 
equipment must be designed and a fuel selected to suit the individual 
plant with its distinctive conditions. The average heat-treatment 
shop needs a drastic awakening from the lethargy of " cut-and- 
dried " systems, poorly designed and " home-made " furnaces, in- 
efficient treatment and handling of products. 

Quality of Product vs. Cost— Quality of product and cost of 
manufacture are the basis of heat-treating operations. Quality of 
product covers the proper heating of the material to meet the met- 
allurgical requirements, and its physical condition to meet the 
mechanical requirements. Cost. of manufacture includes the cost 
of fuel, power, labor, special equipment and material, such as boxes, 
tools, quenching fluids, etc., as well as fixed charges on the equip- 
ment, floor space, etc. Many of the mistakes that have been made in 
heat-treatment installations are due to the fact that the problem has 
been considered from the standpoint of fuel alone or of the first cost 
of installation. Such a view is short-sighted, for the cost of fuel 
alone makes up but a comparatively small part of the total produc- 
tion cost. But when these items are considered in their proper place 
with the other items of operating cost and with the proper inter- 
pretation of the relation of these to the cost of the finished product 

16 



HEAT GENERATION 17 

it will generally be found that the cost of fuel and the cost of installa- 
tion become of secondary importance in measuring or setting a stand- 
ard of excellence to which the product must conform. In other 
words, the ultimate aim of any heat-treating process, from the 
economic standpoint, is to obtain the best heating of the product at 
the least total cost. 

The Standard Heating Unit. — There is no definite standard 
employed for the measurement of production cost in industrial heat- 
ing, as with power or light, because the conditions are continually 
varying and there is no one definite point or method to determine 
the cost. In power the test is the cost per brake horse-power hour 
at the shaft of the machine, irrespective of the purpose of the appara- 
tus. In light the test is the cost per candle power hour, irrespective 
of the fuel employed or the means of utilizing or applying it. In 
electricity it is the kilowatt hour measured at some definite point. 
The nearest approach we can make to a standard for the commercial 
determination in industrial heating is to suggest — " the cost per unit 
cf quantity of given quality." This is somewhat indefinite and dif- 
ficult of location, owing to the many different standards for quality 
and the great latitude in furnace design which affects the elements 
entering into the cost of production above noted. 

Such a standard involves much more than a mere consideration of 
the technique of combustion and other thermal conditions that are 
usually followed and considered as final. It means that the cost of 
finished product is paramount, regardless of fuel cost. And it 
makes a point of considering first of all the application of heat to 
the stock, and then an efficient method of handling that material. 
Working along these lines has produced real results in heating effi- 
ciency (if such a term is permissible) with oil fuel; the gas fraternity 
is beginning to recognize the basic truth of the statement that fuel 
cost does not determine heating cost; and the electrical men are 
slowly falling in line. Each fuel has limits within which it can be 
used, and these are determined by the nature of operations and local 
plant conditions regardless of fuel cost. 

Heating. — Any consideration of industrial heating must neces- 
sarily take into account the right fuel and its proper application, a 
suitable furnace design and construction, and a proper layout of 
equipment and efficient handling of materials. Although each of 
these propositions is, in a sense, distinct, it is obvious that each 
involves and must be co-ordinated with the others. Similarly, the 
broad subject of heating must deal with: 



18 STEEL AND ITS HEAT TREATMENT 

(1) The generation of the heat — the fuel and method of burning it; 

(2) Equipment for employing the heat — the furnace, its design, 
construction and operation; 

(3) The application of the heat — the uniform heating of the stock; 

(4) The utilization of the heat — conserving unnecessary losses in 
spent gases, radiation, etc. 

Comparative Fuel Costs. — The comparison of initial fuel costs 
is always an interesting subject, but unless it is carefully amplified 
and taken only in " small doses," it is apt to be misleading. All of 
the factors given in the previous paragraphs must be considered in 
the selection of any fuel, for, after all is said and done, it is the 
cost of heating as shown by the finished product, and not the 
B.T.U. cost of fuel, which counts the most. 

The chart given on page 19 illustrates graphically the commercial 
relationship between fuels, based on their heat unit cost. 

This chart facilitates the determination of the cost per million 
B.T.U. 's of commercial fuels at different prices; the relative prices 
for different fuels at a definite price for one fuel or per million B.T.U.'s 
and so forth. 

To illustrate — the cost per million B.T.U.'s would be: at 3 cents 
per gallon for fuel oil — 21.5 cents; at 20 cents per M for 1000 
B.T.U. natural gas— 20 cents; and at $5.00 per ton for 12,000 B.T.U. 
coal — 20.8 cents. 

Again, at $5.00 per ton for 12,000 B.T.U. coal, the relative prices 
for the other fuels, keeping the same B.T.U. cost, would be — $5.80 
per ton for 14,000 B.T.U. coal; 21 cents per M for 1000 B.T.U. 
natural gas; 12.6 cents per M for city gas; 2.94 cents per gal. 
for oil; 2.6 cents per M for 125 B.T.U. producer gas, etc. 

Further- — at an assumed cost of 30 cents per million B.T.U.'s, 
the relative prices for various fuels would be — 4.2 cents per gal. for 
oil; 5 cents per M for 165 B.T.U. producer gas; 9 cents per M for 
water gas; 18 cents per M for city gas; $6.00 per ton for 10,000 
B.T.U. coal, etc. 

The B.T.U. Value. — The fanacy of selecting a fuel merely by 
its B.T.U. value alone, however, should be self-evident. To illus- 
trate, take the case of anthracite coal: when broken into pieces the 
size of a man's fist it is used in the ordinary hot-air " heater " or 
house furnace; when crushed to a smaller size it is used in the kitchen 
stove; crushed to rice-size it may be used for forced-draft boilers; 
pulverize it to a dust and it may be used in the " powdered coal " 
systems. But would the furnace size be suitable for the kitchen 



HEAT GENERATION 



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FURNACE ENGINEERS 
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COPYRIGHTED 1914 
























































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SO STEEL AND ITS HEAT TREATMENT 

stove, or would stove coal satisfy the conditions necessary for 
the latter systems? Assuredly not; and yet the B.T.U. value 
of the coal remains absolutely unchanged. In other words, it is 
not the cost of the B.T.U. 's in coal, but all in the manner of its use 
and the equipment provided for the purpose. 

The Combustible Mixture. — Again, the so-called B.T.U. compari- 
son of fuels, which has long been generally accepted as standard, 
is misleading. In fact, it is not the B.T.U. value of the fuel, but 
the B.T.U. value of the combustible mixture that counts. There is 
not as much difference in the heating value of the combustible mix- 
tures of the various gases as there is between the heating value of 
the gases themselves without considering the combustible mixture. 
For example, if city gas were assumed to be 600 B.T.U. and producer 
gas 120 B.T.U., the heating value of the gases would generally be 
considered in the proportion of five to one; and many comparisons 
are made upon this basis. In practice, however, whether it be in 
an internal combustion engine or in a furnace, the actual values of 
the fuels are not anywhere near the ratio of five to one ; if they were, 
then an engine of given size cylinder which would develop 100 H.P. 
with city gas would only give 20 H.P. with producer gas. This 
comparison most people would agree is ridiculous and unreasonable 
on its face, in the light of practice in power; and yet the same com- 
parison frequently is drawn when considering heat, although the 
determinative conditions exist in one case as in the other. 

The Right Fuel. — The generation of heat for heat-treatment 
purposes involves the proper application of the right fuel. Super- 
ficially the choice of fuel appears to be a simple one. Too often, 
however, we are prone to select off-hand some fuel such as coal 
because the initial cost is low in comparison with oil, or oil because 
its initial cost is lower than city gas, or, perhaps, natural gas if it is 
near at hand, or even electricity just because it sounds attractive 
and is easily controlled. Although each of these in its proper sphere 
would be the logical source of heat, as a general proposition no one 
fuel is appropriate to every case. Neither low initial cost, nor local 
supply, nor ready control sums up the situation. These are but 
factors in the case as a whole, and the value of one fuel cannot be 
measured by its use in the way another fuel would be used. There is 
always a right fuel for the particular work in hand, so that each prob- 
lem must be thoroughly studied and understood if the best solution is 
to be had. 

" Fuel Efficiency." — There is, or at least should be, no argument 



HEAT GENERATION 21 

on the fuel question. The relationship of the various fuels is fixed by 
physical law and commercial conditions. The term " fuel efficiency" 
is not employed in power or in house or factory heating, and it should 
not exist in industrial heating. Generally speaking, it may be assumed 
that any difference in results between oil, or gas, or electricity for 
heating furnaces should be attributed to the manner in which the 
heat is applied to the stock or the nature of the furnace equipment 
employed and not to any inherent advantage in one form of fuel 
over the other. It is not proper to say that oil is cheaper than 
electricity or coal or gas, or vice versa, or that a higher price fuel can 
displace another because the former is " more efficient." There is 
nothing in the argument that one fuel will do more than another 
without reference to the manner in which it is to be employed. 

Thus, in industrial heating, it is the nature of the operation and 
the matter of applying and utilizing the heat that counts, and not 
the fuel alone. The manner of its use may be efficient or the means of 
employing or utilizing it, but certainly not the fuel itself. It can be 
said that electricity in a given type of furnace will produce a better 
result than oil, gas or coal in a given type of furnace, but it will be 
noticed in so doing that it is the difference in the manner of applying 
or utilizing the heat, which is equivalent to a difference in furnace de- 
sign, that determines the result, and not to any advantage in one form 
of fuel over another. The term "fuel efficiency," therefore, is mislead- 
ing in that it is employed to express a condition that does not exist. 

Fuel vs. Operations. — It may be said that the extent of the heating 
operation and the plant conditions more or less determine the fuel. 
It might be good practice to employ city gas for the annealing of a 
small quantity of wire in a loft building; with a larger quantity and 
suitable working conditions the fuel might be oil; but if the opera- 
tion is conducted on a still larger scale, as in a rolling mill where the 
size and type of furnace will so permit, then the fuel might be pro- 
ducer gas or coal. Thus city gas at $1.00 a thousand for many oper- 
ations might be recommended in preference to oil or gas at 1 cent a 
gallon, and electricity at any price in preference to oil or gas at any 
price. In annealing, for instance, we might use oil for certain results 
and in same room use coal for annealing the same metal, but for 
different results. Thus the nature of the operation more or less 
determines the working limits, regardless of fuel cost. 

The " Fluid Fuel." — We can even go a step further and state 
that no one fuel has a monopoly on uniform heating, or control, or 
economy. At the present time there are many furnaces of good 



22 STEEL AND ITS HEAT TREATMENT 

design, operating on cheap coal, which produce a better quality of 
product at less cost and maintain more uniform temperatures with 
more accurate control, than other furnaces of poor design built 
for the same purpose, using either oil or gas. All other things being 
equal, a " fluid fuel " such as gas or oil (as distinguished from a solid 
fuel) is generally to be preferred, as it lends itself more readily to 
accurate control. Ordinarily it would be assumed that a fluid fuel 
would permit of greater flexibility of operation than a solid fuel, and it 
invariably will when all other conditions are equal. However, as pre- 
viously noted, there are many cases where the advantages of the more 
flexible fuel are lost with inefficient means of utilizing it. But it is mis- 
leading to couple any fuel with the term Uniform Heating, or Control, 
or Economy, without specifying or qualifying the manner in which it 
is to be used. It is the manner of applying and utilizing the heat and 
not the fuel which determines the successor failure of the operation. 

Even the selection of a fluid fuel is dependent upon conditions 
ether than heating value or composition of the fuel itself. For 
instance, we might consider a very small furnace for annealing, tem- 
pering or hardening small pieces of steel. There would seem to be 
no question but that gas would be the fuel most generally preferred. 
But even the selection of the gas itself would be dependent upon con- 
ditions Other than that of temperature control. To illustrate : While 
one might use either producer gas, water gas, city gas or natural 
gas in the case just mentioned, it would not follow that each of the 
fuels could be considered if the operation was one requiring a very 
high temperature, such as welding- The same conditions would hold 
good if the operation was one requiring a very low temperature, such 
as japanning. If, however, the operation were to be conducted on a 
larger scale, and one involving the use of a large furnace in which 
there would be ample room for combustion and distribution of heat, 
there would be brought into competition with these gases a liquid 
fuel in the form of oil, which, by reason of the latitude afforded in 
furnace design, could compete from the standpoint of temperature 
control, and it would very likely compete in the matter of cost. 

Influence of Working Conditions.— There is generally a confusion 
between the terms Uniform Heat Distribution and Uniform Fuel 
Distribution; the two do not necessarily go together, although at 
times they do. For instance, in the case of japanning, requiring a 
low temperature, particularly in small and medium-size ovens, a fluid 
fuel in the form of gas is generally preferred. The reason for this 
is that the fuel may be applied on all sides of the oven and burned in 



HEAT GENERATION 23 

small quantities at different points. This could not be as readily 
done with a liquid fuel like oil, which by reason of its great calorific 
power and difficulty of control when burned in very small quantities 
in a limited space, would prohibit a distribution of the fuel in the 
manner usually provided for gas. If the oven were large and there 
were plenty of room for combustion and distribution of heat through 
flues or radiators, it would be possible to approach the same con- 
ditions of uniform heating as far as the stock is concerned, without 
uniform distribution of fuel. In one case the result of heating the 
material is accomplished by distributing the points of heat genera- 
tion, while in the other it is produced by localizing the point of heat 
generation and distributing the heat after it is generated. This 
goes to show why working conditions are, as a rule, determinative 
not only of the fuel, but of the manner in which it may be employed. 
Also, to show that the conclusions formed in one case may be reversed 
in another, when a change in working conditions makes it either pos- 
sible or desirable, even though there be no change in the composition 
or price of the fuels themselves. 

Cost Factors with Producer Gas. — The question of regeneration 
or recuperation is usually misunderstood. Recuperation is generally 
commercially desirable on general principles, but there are times 
when it is physically necessary, irrespective of the economy of the 
operation. For instance, neither hot nor cold producer gas is suit- 
able for high forging or welding heats in furnaces without pre- 
heating the air, or the gas, or both, by regeneration or recuperation, 
because the heating value of the fuel is so low. Because of their 
greater heat value, water gas, natural gas, city gas or fuel oil might 
be used for operations with which producer gas could not be con- 
sidered, for the reasons given above. But if the furnaces were large 
and the principles of recuperation could be employed, then the 
producer gas could compete, and the extent to which it could compete 
would be determined by fuel cost coupled with the manner in which 
the fuels were employed. Very often a comparison is made with 
producer gas used in this form against the other fuels to show that 
the gas would be the cheaper. But this in itself is not conclusive 
unless the determination is based upon the results secured by the 
use of the other fuels in a manner which would involve recuperation 
in substantially the same manner, and in this way take advantage 
of every possible saving. Producer gas, as a rule, would show the 
highest efficiency from the standpoint of recuperation, because th& 
volume of the inert or non-combustible gases is greater; but it doe? 



24 STEEL AND ITS HEAT TREATMENT 

not follow that this efficiency of recuperation is in itself determinative 
of the fuel. 

Even though fuels were compared on the basis of heating value in 
the manner above indicated, it would not follow that the result in 
the furnace would be as conclusive from the standpoint of industrial 
heating engineering as it would be from that of fuel value alone. The 
reason for this is that some fuels by their very nature are adapted 
better than others to the great variation in working conditions govern- 
ing the same manufacturing process in different plants. The con- 
ditions that hold good with an open-hearth furnace do not obtain in a 
comparatively small forging or heat-treating installation made up of 
scattered units. A clean gas that could be delivered to a furnace 
through pipes, and thus distributed to scattered units thoughout a 
plant, would cost more on a heat unit basis than hot, unwashed pro- 
ducer gas. But the lower heat-unit cost of the hot gas at the pro- 
ducer may be offset by the more expensive fuels when compared at 
the furnace, on account of the losses in heat, deposits of tar, labor 
of cleaning and fixed charges incident to the use of brick flues with 
hot producer gas. There is no question that the B.T.U. cost 
at the producer is less with hot gas than with clean washed gas 
because of the heat loss in the latter in the process of scrubbing, drying 
and cleaning; on this comparison of B.T.U. cost at the producer 
much has been written and many claims have been made for superi- 
ority one way or another. 

It must be borne in mind, however, that in industrial heating it is 
not the cost at the producer or at the point of fuel supply that counts, 
but rather the cost of the finished product at the delivery end of the 
furnace. This " heating cost," as it might be termed, takes into 
account not only the fuel at the furnace, but the utilization of the 
fuel in the furnace, which naturally involves a consideration of fur- 
nace design and layout. The fuel, at best, is secondary and not of 
the paramount importance frequently considered. 

In the cases of such artificial gases as producer gas, water gas, etc., 
the cost of the necessary plant equipment for their manufacture, 
besides interest and depreciation, must also be figured in with the 
cost of the gas. The question of floor space occupied is also very 
important and it happens frequently that from the manufacturing 
point of view the value of this space and the building necessary to 
house the gas plant is greater than the value of the gas plant from 
the standpoint of lower fuel costs. It is evident that the heat treat- 
ment plant and the space available must be of considerable size to 



HEAT GENERATION 25 

warrant a large initial plant investment for the manufacture of such 
gas. 

Fuel Oil. — There is probably no one fuel that has been more 
abused than fuel oil. Its great concentrated heat value and flexibility 
of application and control have been, commercially, its greatest draw- 
back, for the reason that these advantages have permitted the applica- 
tion of the fuel in a haphazard manner by people who seem to be 
satisfied so long as it was burned and made heat in some form or 
other. Even many of the manufacturers of oil-burning equipment 
are not entirely free from this criticism. Much of the competition 
which has been held against fuel oil, and comparative statements of 
operating costs that have been made are based generally upon an 
inefficient use of the fuel in improperly designed and operated fur- 
naces. 

The majority of heating equipment installed with oil is lacking 
in some of the elementary essentials necessary for good combus- 
tion, and in this respect are not anywhere near as efficient as an 
ordinary kitchen stove. There seems to be an absolute disregard 
of the fact that it is just as necessary to control all of the air entering 
into a furnace and the gases leaving the furnace with oil fuel as it is 
with gas or coal. Most city gas equipment has this provision, as 
have boilers or ordinary stoves, but it seems to be entirely lacking 
with most oil-burning equipment, although the reasons for it are just 
as important in one case as in another. The common practice is 
to inject oil through a hole in the side of a furnace; and many opera- 
tors think that because there is a valve on the oil and air lines that 
they consequently control the air. This, however, is not always 
true, for the reason that in many furnaces by far the greater propor- 
tion of air required for the combustion of the fuel is " induced " by 
the force of the blast and does not pass through the burner itself. 
It has been by reason of such conditions that oil has been abandoned 
and given a bad name in many places where the conditions would be 
reversed if it were properly handled. 

Oil Burners. — There is altogether too much importance attached 
to oil burners, both by manufacturers of such appliances as well as 
by users. Too many people have the idea that all that is necessary 
to burn oil is to buy an " efficient " burner and build a furnace 
around it. But there is really no such thing as an oil burner in the 
sense usually taken for a gas burner. The very term is a misnomer, 
as the oil burner is nothing more than a valve and its efficiency is 
mechanical and not thermal. In fact, the majority of oil burners 



26 



STEEL AND ITS HEAT TREATMENT 



are not, properly speaking, mixing valves, as most gas burners are, 
for the reason that the most successful burners are nothing more 
than valves which introduce fuel and air into the furnace in pro- 
portions fixed by the operator, the actual mixing taking place in the 
furnace and not in the burner itself. It does not matter as much 
how the fuel is delivered at the furnace as what is done with it after 
it is delivered in the furnace. 

This influence which the design of the furnace — that is, the method 
of using the fuel in the furnace — has upon the economy of heating, 
regardless of the burner, is well brought out by the following chart, 
to accompany Figs. 1, 2 and 3. 





Fig. 1 


Fig. 2 


Fig. 3 


Metal heated 

per hour 


287 pounds 


576 pounds 


1450 pounds 


Increase of metal 
heated per hour 




100 per cent. 


405 per cent. 


Fuel oil burned 

per hour 


6.41 U. S gals. 


3.64 U. S. gals. 


3.48 U. S. gals. 


Decrease of fuel 
oil burned per hour 




43.3 per cent. 


45.7 per cent. 


Metal heated per 
gallon of fuel oil 


45 pounds 


158 pounds 


416 pounds 



In each case the pieces heated were of the same size and material 
and the lightest in individual weight of their kind. The figures 
represent three different types of rotary heating furnaces doing the 
same work. These three furnaces were operated with the same 
burner, the same fuel oil, the same pressure for atomizing, the same 
air for combustion, the same material, the same temperature, at the 
same time, and by the same men. This then illustrates the point 
that it is the furnace and not the burner alone which produces the 
desired results. 

Electricity for Heating, — The advantages of electricity as a source 
of heat compared with combustible fuels, or vice versa, are determined 
only by the nature of the operation regardless of fuel cost. Each 
form of energy has its own field of use. When it is considered that 



HEAT GENERATION 



27 



Fig. 1. — Externally Fired Cast-iron 
Tumbling-barrel Furnace. 




Fig. 2. — Externally Fired Cast-iron Helical-cylinder Furnace. 



Steam 
or Air - 



^ Pyrometer Connection 

-. Discharge Hood 



Charging Drum 




Fig. 3. — Internally Fired Tile-lined Helical- cylinder Furnace. 



28 STEEL AND ITS HEAT TREATMENT 

gas and fuel oil under proper application are continually being used 
within temperature limits of 10° F., it must be evident that electricity 
must offer some strong, indirect advantages in order to be given 
serious consideration. On the other hand, there are many cases like 
the open-hearth, reverberatory and other forms of heating where the 
limit of efficiency in applying heat to the charge has been about 
reached with fuel, owing to the limitations in latitude for radical 
changes in furnace design. It is in such operations where electricity, 
by reason of a better method of applying heat to the stock, can over- 
come the disadvantage of higher fuel cost, with less actual energy for 
the operation. Thus, we might designate also as good examples 
where electricity might be advantageous certain forms of welding, 
brazing and similar heating operations requiring short or localized 
heats, melting, special laboratory operations, and others where a 
distinctive application of heat to the charge — resulting in some metal- 
lurgical or manufacturing advantage — might be more important 
than fuel cost. 

In general, it may be assumed, in connection with heating oper- 
ations which, by their nature, permit of the use either of fuel or of 
electricity, that the high energy cost of the latter is to its disadvan- 
tage. Comparing the operation of an efficient electric furnace 
with that of an inefficient gas or oil furnace is misleading for the 
reason that it frequently leads to a misunderstanding of fuel values 
and in the long run reacts unfavorably. Proper consideration of the 
real standard of heating, previously explained, does not recognize 
any competition between fuels when one will produce a necessary 
result which another fuel will not produce, no matter what their 
comparison on a heat-unit basis may be. 

When this standard is properly recognized and accepted it nat- 
urally and properly will lead to a further development of electric 
furnaces. The corresponding development of other types, coupled 
with manufacturing requirements and conditions, will establish the 
condition by which the manufacturing result sought for will of itself 
dictate the furnace and the means of heating it, whether it is elec- 
tricity or fuel, and regardless of cost. 

" Quality of Heat." — Much has been said about " the quality of 
heat "; but as heat can differ only in the degree of temperature, such 
a statement must, therefore, refer to the atmosphere in the furnace. 
One of the recent books on the subject of heat treatment of steel 
contains the following statements: 

" Until recently, the only known way of producing heat of the 



HEAT GENERATION 29 

required intensity was by combustion — the burning of some fuel. 
The attendant disadvantages of this are well known. The crude 
open coal forge is capable of heating the steel, but leaves much to be 
desired as regards the quality of the heat, its uniformity, and the 
temperature control. In order to produce heat at all, the carbon 
in the coal must be combined with the oxygen of the air, and a 
strongly oxidizing flame is unavoidable. The steel exposed to this 
action, or to the inevitable results of it, suffers accordingly. The 
coke-burning furnace offered some improvements, but only in detail. 
Now there are highly perfected furnaces for burning oil and gas, 
and some of these offer still further advances, but the principle at 
the basis of all of these is the same — there must be a ' burning ' 
process to produce the heat; oxidation must be present with all fuel- 
combustion furnaces. 

" Through what means, then, may we obtain the proper quality 
of heat, uniformly applied, and of the right degree? The electric 
furnace for the heating of steel brings the answer. It overcomes 
most of the objections to the ' combustion process ' by introducing 
a new principle." 

Further on the statements are made: "The atmosphere in 
the heating chamber of the electric furnace is inherently ' reducing ' 
in its nature, due to the fact that the hot carbon plates absorb all 
of the atmospheric oxygen. By raising the door slightly, and open- 
ing the draft-hole at the rear, a slight current of air may be admitted 
which will counteract this tendency. Leaving the door open slightly 
more would allow an excess of air to enter, so that an oxidizing 
atmosphere could be produced. Between the extreme points fine 
shades of atmospheric conditions can be obtained. Thus the qual- 
ity of the heat can be absolutely and easily regulated." 

Furnace Atmospheres from Combustion. — Commenting on the 
above, the question of atmosphere in the heating chamber is one of 
operation of the furnace, assuming the proper furnace design, and 
simply comes down to the relation between the fuel and the air 
supply. In the design and operation of the best types of heating 
furnaces it is the aim to produce an atmosphere, under pressure, 
which virtually contains no oxygen and only a very slight amount of 
reducing elements. Take for example the under-fired type of fur- 
nace, properly designed and operated. The actual combustion of the 
fuel takes place in the separate chamber under the hearth where the 
amount of air taken into the furnace is just sufficient to produce 
perfect combustion of the fuel. By the time the hot gases actually 



30 STEEL AND ITS HEAT TREATMENT 

reach the heating .chamber they are thoroughly mixed and, with 
proper design and operation of the furnace, a pressure is built up. 
This pressure stops any inflow of free oxygen through the doors, and, 
otherwise, surrounds the steel with the neutral atmosphere of hot 
gases. That these hot gases may actually contain no oxygen and 
very little reducing vapors is well shown by the following analysis 
of gases taken from near the hearth of an improved type of furnace : 

Per Cent. 

Carbon dioxide 12.5 

Oxygen 

Oil vapors 2.1 

Carbon monoxide 

Nitrogen 85.4 

From a study of this analysis of chamber gases which were taken 
under ordinary operating conditions and without the operator know- 
ing that anything unusual was expected of him, it certainly cannot be 
said that the " combustion process " produces, under proper condi- 
tions, an atmosphere which is unsuited to the general run of heat 
treatment work. 

Furnace Atmospheres with Electricity. — In contrast with this, 
consider the effect taking place in the electric furnace previously 
referred to. In this case the heat is virtually supplied outside of, 
and through, the walls of the chamber. Free access of unaltered 
atmospheric air to the inside of the furnace exists, and coming in 
contact with the heated steel results in a very rapid oxidation of the 
metal. An attempt is sometimes made to remedy this condition 
by introducing charcoal into the chamber with which the oxygen 
is supposed to combine to form an inert atmosphere of carbon diox- 
ide, or even by exposing the carbon electrodes of the electric element 
and allowing the oxygen present in the chamber to attack them and 
thus consume the free oxygen. It must be. realized that if this 
elimination of the free oxygen thus occurs it must take place in the 
furnace itself and in the chamber where the stock is being heated 
and the steel is, therefore, more or less exposed to oxidation. 

The argument previously advanced that the atmosphere in the 
electric furnace may be controlled by slightly raising the door and 
opening a vent in the back of the furnace is entirely contrary to 
recognized heating principles. The cost of the heating itself advances 
when this takes place because cold air is entering through the front 
and the heat is allowed to flow out through the vent. Such a propo- 



HEAT GENERATION 31 

sition is analogous to the effort of trying to heat a house in the winter 
with the doors open. 

The " Fuel Question." — The real problem involved in the so- 
called " fuel question " is not the cost or price of fuel, but rather the 
selection of that particular fuel best adapted to the requirements of 
the individual case and providing the means necessary to decrease 
waste in the use of that fuel. The manufacturer rarely can control 
the price of fuel, but he usually can control the amount to be paid 
for — and that amount naturally decreases as the waste decreases. If 
the energy that has been lost in consideration of " fuel values," " cost 
of fuel," and other phases of this problem was directed intelligently 
towards the selection of proper equipment and fuel suited to it, and 
to decrease the waste of such fuel, our manufacturing methods, cost 
of operation and conditions in general would be more attractive. 

The relation of the fuel question to the progress which has been 
made in generating power, lighting or heating buildings, and in the 
field of traction with steam or electric locomotives, ship propulsion 
and motor cars, clearly indicates the relation of fuel to the real 
problem of industrial heating. The present general inefficient state 
of the art, the slow progress which has been made, the usual lack of 
consideration given to it and the apparent room for improvement 
indicate the necessity for considering the fuel only in relation to 
the problem as a whole. 

Fuel is but one element in the problem of power, heating buildings 
or traction, and it bears the same relationship to the industrial 
heating problem. In the latter field, however, the necessity for 
variation in selecting, and the difficulty of standardizing, the fuel is 
more pronounced, owing to the much greater variation in manu- 
facturing requirements and plant conditions and the latitude in 
furnace designs to meet them. 

The real question is the cost of heating, of which — as with power — 
the cost of fuel is but a part. It is the total operating cost that 
determines the result, and this generally is greatly influenced — if not 
actually determined, by the efficiency of the design and operation of 
the equipment, and not by the fuel itself. To sum up, it may be 
said that: 

That fuel should be selected which best is adapted to the heating 
requirements and plant conditions and which, with proper operation 
of the best furnace equipment, is likely to cost the least from the stand- 
point of finished product, regardless of its cost on a heat-unit basis. 

Further discussion of the heating problem is given on page 69. 



CHAPTER III 
HEAT APPLICATION 

Uniformly Heated Product. — Since the standard of heating is 
measured by " a unit of quantity of given quality," and since the 
quality must be determined in terms of finished product, the prac- 
tical test of the success of any heat-treating process must depend 
upon the uniformity in quality of the finished product. This, in 
turn, is defined in terms of " uniformly heated product." The 
underlying principles of "heat application" must be ' associated, 
therefore, with " uniformly heated product," and what is most 
required by engineers at this time is ,a better and more complete 
expression and understanding of the facts which deal with this, and 
generally are not considered. 

" Uniformly heated product " involves a consideration of: 

1. The temperature or degree of heat; 

2. The time required for saturating; 

3. The surface ex-posed to the heat, which determines the rate of 
heating and influences the period of saturation and degree of uniformity ; 

4. The mass l to be heated, which determines the period of saturation 
and the manner of applying heat to the steel. 

It is to be understood that the terms " mass " and " surface " 
must be considered in their relation to " cooling " as well as 
" heating," because " uniformity in cooling " is just as much a part 
of the process as " uniformity of heating." 

Temperature Variation. — Aside from the exact degree of heat 
necessary to bring about the required specific molecular changes in 
the pieces or mass to be heated, and which is a strictly metallurgical 
problem discussed elsewhere, the temperature problem in relation 
to heat application and the finished product is more directly con- 
cerned with the temperature variation around the mass to be heated, 
and not merely with the ability of a furnace to maintain a uniform 

1 The term " mass " is intended to apply to the individual piece or the charge 
in the chamber at one time, and is to be considered apart from the term 
" quantity." 

32 



HEAT APPLICATION 33 

temperature at any one point or at any number of points without 
reference to the mass. 

The usual consideration of furnace equipment for heat-treatment 
operations is based on requirements of " accurate temperature," 
" heat control," " temperature variation," etc., with the result 
that the trade literature — which is the catechism of many, if not of 
the majority interested in the work — has developed a standard of 
heating which is altogether too low and must be superseded by one 
based on a broader view of the problem in order to effect greater 
progress. 

The existence of such a standard is probably due to the reasoning 
— " that a uniformly heated piece naturally involves a uniform 
temperature within the furnace, and the less the variation in tem- 
perature the better the results will be; therefore, in order to produce 
a uniformly heated product it is necessary to employ a furnace that 
will produce a uniform heat with minimum variation of temperature." 
The natural result of such reasoning has been a development of 
pyrometers to indicate the variations in temperature, and much 
discussion of the relative merits of the different designs of furnaces. 

Much of this has been directed towards the claims made for the 
furnaces supported with evidence in the form of pyrometer charts, 
heat logs, and other data incident to the indication of temperature, 
tending to show the ability of the various designs to meet exacting 
heating requirements. This is all right as far as it goes, but it does 
not go far enough. It may be considered as " evidence " of a heat 
condition in some part of a furnace chamber, but not necessarily 
as " proof " of a uniformly heated product within that furnace 
chamber. If it were otherwise, then we would have not to deal so 
often with variations in the finished product without any apparent 
variation in the indicated temperature. Heating a furnace chamber 
uniformly, and uniformly heating a product within that furnace 
chamber, are two distinct operations. The former must accompany 
the latter, but the mere indication of the former does not by any 
means prove the existence of the latter. It does not follow that the 
temperature variation indicated in any two points in a chamber 
when empty will be the same as that indicated when the chamber is 
partially filled, or more particularly, when it is filled to full normal 
capacity. 

We may now add another rule to those previously noted on page 
32 and the state that: 

5. The real test of a furnace for a given operation from the stand- 



34 



STEEL AND ITS HEAT TREATMENT 



point of uniformly heated product is not the temperature variation 
when the chamber is empty or partially filled, but the temperature varia- 
tion around the mass or pieces to be heated when the chamber is loaded to 
full working capacity. 

This is a simple fundamental rule which, when considered with 
reference to a given operation, illustrates why it is possible to secure 



nnnnooooon oononnonn 



J I 



(a) 



OOCUUUUUOOUUUOUUUUO 



(&) 



onnnnnnnnnnnnn o.innn 



(c) 




nHXDD 



onnon onnncioononnnnn 



(e) 



UHOUVUll oil 



EEHED 



if) 



Fig. 4. 



better results in finished product with one type of furnace having a 
distinctive method of applying heat to the steel over another, without 
any apparent difference in the indicated variation in temperature 
at any given point or plane in the chamber. 

To illustrate: Let us consider two gas-fir d bake-ovens of the 
same size, each designed to accommodate six cakes of a given 
size, the essential di . erence between the two being that one 
is heated by gas jets at the bottom, as in Fig. 4 (a), and the 



HEAT APPLICATION 35 

other by gas jets at the top, as in (6), the problem being the 
uniform heating or baking of the six cakes to be placed on the 
tray x. 

If a thermo-couple should be introduced at any two points on the 
tray x in the empty ovens, it will be found that the heat will be 
fairly uniform with either design, even though with one design the 
actual oven temperature may be different from the other. It is 
reasonable to suppose that one small cake, as in (c), would compare 
favorably with (d) , as there is ample room for circulation of the heat 
from one side to the other. We have in one case (a or 6) an indi- 
cation of a uniform chamber temperature, and in the other (c-d) 
an indication of a uniform chamber temperature and of a uniformly 
heated product. 

Now suppose that each oven is filled to normal capacity, as in 
(e) or (/). If the space between the pieces is small, it is very likely 
that, in a given time, a given temperature will produce a color on the 
side of the cake nearest the fire that will be different from the color 
on the opposite side. It is also likely that the variation between 
any two points on the same side of the tray will be very slight, but 
that the variation between the under and upper sides of the tray 
will be considerable. This indicates the possibility of turning out 
a product not uniformly heated from a chamber that may be uni- 
formly heated when empty, or but partially filled, and at the same 
time constantly indicating a uniform temperature on any lateral 
plane. It also illustrates how weak and irrelevant such claims as 
previously noted are to the real question — the uniformity of the 
heated product. The very manner of placing the stock in the 
chamber may a.Tect the final result. Uniform temperature in the 
chamber is a part but not all of the process. 

Heat Application and Furnaces. — Heating a piece of steel, boiling 
a cup of water or baking a potato are alike heat-treating operations, 
and, in so far as each leads to an absorption of heat, they are com- 
parable. A cup of water will boil much sooner if the heat is applied 
from the bottom rather than from the top downwards. In the ordi- 
nary gas stove the oven is heated from jets below, while these same 
jets deflected downwards heat the broiler from above; a potato in 
the oven is heated evenly through to the center without unevenly 
heating the outside, but a potato placed on the broiler may be burned 
to a crisp on the top while the center and bottom remain hard and 
uncooked. 

Many carburizing and annealing boxes give evidence of having 



36 STEEL AND ITS HEAT TREATMENT 

been " broiled " rather than " baked," and the variations in the 
surface structure and test pieces of the finished product, without any 
apparent change in the pyrometer reading, will illustrate the weak- 
ness of the methods of heating and that the actual operation of fur- 
naces is not based upon the principles previously noted. Boys still 
roast potatoes in open bonfires; men still heat steel in open smith- 
fires; and good results are obtained within narrow limits when care 
is taken to insure the same application of heat to all of the surface. 
Such methods are unsuited for the production of uniformly heated 
product in large quantities, and illustrate the necessity for consider- 
ing the action of the heat on the mass to be heated as well as efficient 
equipment to accomplish this result at reasonable cost. An ineffi- 
cient plant means that the product seldom reaches and never main- 
tains the standard to which it is entitled, and that the cost of opera- 
tion far outweighs any difference in the first cost of plant. 

To make results harmonize with the requirements in each case 
demands furnaces of the best possible construction, fuel that would 
cost the least viewed from the standpoint of finished product, and it 
further requires that these furnaces be so arranged with reference to 
the size and shape of the chambers and openings and such routing 
and handling methods devised that the material may be heated and 
handled with the least labor and loss of time. Only when this har- 
monious combination of suitable furnaces, right fuel, proper furnace 
layout and efficient material handling conditions has been secured 
can there be accomplished the best heating of a product at the least 
total cost. 

The " One " Furnace. — One often reads of the claims of furnace 
manufacturers that this or that furnace is the " only one " for heat 
treatment. This is all wrong, for there is no one type of furnace for 
heat treatment any more than there is one type of building for machine 
operations or one type of automobile for all transportation require- 
ments. There are certain principles of construction, heat genera- 
tion, application and utilization that, when properly combined, 
make up the right furnace; but it is always the local shop conditions 
that determine how these combinations should be effected. There 
is as much more experience and skill required for the determination 
of a furnace design than in building it, as there is between an architect 
and builder or carpenter in the design and erection of a building. 

Furnace Guarantees. — One would not think of asking a stove 
manufacturer to guarantee good cooking with his stove, and it is as 
unreasonable to expect, as it is foolish to offer, a furnace guaranteed 



HEAT APPLICATION 37 

for good heat treatment without the proper handling. The right 
furnace, like the right stove, only makes it possible ; and it is the man, 
like the cook, that determines the final result. What is most needed 
at this time is a better appreciation of heat application to useful 
work as defined on page 33 and as removed from " combustion," 
uniform chamber temperature, temperature control, fuel economy, 
etc. There should be more study given to the absorption of heat by 
the product and the manner of placing it in the furnace to secure the 
best heating results. 

Uniform Heating. — The problem of applying heat to industrial 
work is but a cooking operation, like the baking of bread; the bread 
or the steel must be heated uniformly throughout. Similarly the 
charge in a heat-treatment furnace may be best heated when it has 
opportunity to absorb heat uniformly from all sides. And as far 
as the heat absorption is concerned it does not matter whether this 
heat is supplied as radiating heat from the lining of the furnace or 
through the direct application of hot gases. Except in the case when 
electricity is used as the source of heat energy, it may be said that the 
majority of commercial heat-treatment furnaces involve the applica- 
tion to the work of hot gases obtained through a combustion process. 
If it were then possible to apply this heat so that the charge would 
be equally and simultaneously heated on all sides the general ideal 
condition would be met. 

Underfiring. — In principle, 1 it is best to apply the heat to the 
bottom of the charge. It is natural law that heat or hot gases tend 
to rise — a very simple and important fact and yet one so frequently 
overlooked in industrial heat application. Further, since in practical, 
everyday work the separation of the charge (in order to provide for 
circulation throughout the entire mass) is more or less reduced to a 
minimum, and which necessarily results in the major part of the charge 
to be heated being near the hearth or even lying directly upon it, we 
may consider that the best construction in general to adopt is to place 
the initial heat where it is most needed — which is under the charge. 
As much advantage as possible is then taken of the natural law of 
hot gases rising in effecting a further application of that heat to the 
sides and top of the charge. This, in effect, is underfiring. 

Simple Underfired Furnace. — A common type of heat-treatment 
furnace is illustrated in Fig. 5. The heat is generated in the com- 
bustion chamber under the hearth, supplying heat to the hearth. 

1 We say this, because we will show later that certain conditions may entirely 
reverse this in practice. — Author. 



38 



STEEL AND ITS HEAT TREATMENT 



The hot gases then rise upon either side of the hearth to the roof, 
where they become mixed or equalized, and then are forced down 
upon the floor of the chamber by the pressure which has been built 





Fig. 5. 



Fig. 6. 



up. In the type of furnace shown it will be noted that the furnace 
door opening is the same height as the roof arch, and that there is 
also a vent located in the roof. Such a design is permissible for 
small furnaces built upon legs to lighten the weight for transpor- 
tation, and where first cost is an important feature. 

Side Ledges. — One of the objections to this common type of 
furnace is the inherent tendency to overheat at the sides of the hearth. 
As the hot gases sweep upwards from the combustion chamber 
towards the roof' the sides of the hearth will become hotter than the 
central area, with the consequent superheating of any material 
placed at the sides of the furnace near the ports. To overcome this 
tendency to localized heating, the sides of the hearth are frequently 
protected as illustrated in Fig. 6. Such an arrangement affords 
plenty of room for circulation at the sides, tends to prevent cutting 
action near the floor line and automatically stops any overloading 
at the sides of the furnace. 

High Ledges. — It was thought originally that the flow and heat 
transfer of the hot gas currents would be in an underfired furnace, 
as in Figs. 5 or 6, from the combustion chamber to the hearth and 
the metal on the hearth, and thence to the roof. It was then thought 
that if the hot gases, after leaving the combustion chamber, could 
be made first to travel to the roof, be thoroughly equalized, and 
then be brought down to the hearth, that a more uniform heating 
of the charge and mixture of gases would result. The furnace in 
Fig. 7 shows an early development of the underfired furnace in an 
attempt to do this. The side ledges were made to reach nearly to 



HEAT APPLICATION 



39 



the roof so that the gases had to go there directly after leaving the 
combustion chamber. Experience with furnaces of the type shown in 
Fig. 7 soon showed, however, that the hot gases — following natural 
law — would first go to the roof whether these high side walls were 
there or not. In other words, they were proven unnecessary. 

The second advantage hoped for through the high ledges in Fig. 
7 was entirely to prevent any localized heating at the side of the 
charge. But it was then discovered that the small opening between 
the top of the ledge and the roof arch tended greatly to increase the 
velocity of the gases as they entered the heating chamber — that is, 
to form a blast action at the top of the chamber. Thus, if the charge 
were anywhere near the height of the door opening, a localized heat- 
ing at the edges and top of the charge would at once result. For 




Fig. 7. 



this and the previously mentioned reasons this type of furnace 
(Fig. 7) is not generally used now. 

Roof Vents. — It will be noted that in the furnaces in Figs. 5 
and 6 there is a vent in the roof arch, and while this is general prac- 
tice, it is not good. One writer, in discussing this point, states that 
" as only approximately 20 per cent, of the air for combustion is 
oxygen, the balance is inert gases which unfortunately must be heated 
to the temperature of the furnace and expelled as quickly as possible. 
In a scientifically designed furnace, this is readily done by the aid of 
the burner. If allowed to pocket or remain stationary in any 
portion of the furnace, the inert gases cause uneven temperatures. 
"... The vents for the escape of . . . the consumed and inert 
gases should always be located in the oven roof or arch." 

In the first place, that writer evidently loses sight of the fact 
that, with perfect combustion, all the gases become " inert " (assum- 
ing that he means a non-supporter of combustion); and that the 
furnace and stock are principally heated by the blanket action of hot 



40 



STEEL AND ITS HEAT TREATMENT 



gases. He admits that these gases are hot, but then reasons that 
because they are hot and inert they must be " expelledas quickly 
as possible." But why throw heat away? In other words, he 




Fig. 8. 



believes in trying to heat his house in winter time by throwing open the 
skylights or windows! He fails to perceive that with an open vent 
in the roof the hot gas currents will tend to short-circuit directly 
from the combustion chamber to the roof and discharge at the max- 
imum chamber temperature. (Incidentally, exactly what part the 
burner — which is merely a valve for injecting fuel into a furnace — 
plays in this ejection is not mentioned.) But by eliminating the 
vent in the roof the gases are given ample opportunity thoroughly to 
mix in the upper part of the chamber and are then forced down as 
a blanket at a reduced velocity upon the stock. This same pres- 
sure, as we have explained in the previous chapter, will prevent cold 
air from the outside from finding its way into the furnace. If 
" pocketing " is feared this may easily be overcome by providing 
some flue outlet on a level with the hearth, as in Fig. 8. 

Vents and Cold Streaks. — The question of roof vents may also 
be approached from a different angle — that they will tend to set up a 
current of cold air through the heating chamber. The natural 
tendency of roof vents is, as we have previously said, to short-cir- 
cuit the hot gases through the roof. This means that there will be 
a pull or suction around the door towards the inside of the furnace 
across the charge and thence to and out through the roof. Cold 
air, with free oxygen, will, therefore, be sucked into the furnace 
and will cause scaling, non-uniform heating and widely variant 
results. The result of these vents is a loss of heat, fuel, and time, and 
destruction of the lining through contraction due to quick cooling 
after the burner has been shut off or when the door is open. This 
condition is emphasized by an unfortunately common type of heating 



HEAT APPLICATION 



41 



furnace shown in Fig. 9; in this instance the heat is supplied from 
above the hearth, and by looking into a furnace of this type a dark 
streak of cold air will be seen to lay directly over the steel 1 on the 
hearth. 




Fig. 9. 



Door Heights.— It will also be noted that in Figs. 5 and 6 the 
height of the door opening is the same as the height of the roof arch; 
this is also objectionable practice under most circumstances. It is 
reasonable to assume that the height of the door opening is governed 
by the maximum height of the charge desired, plus a reasonable 
allowance to facilitate the handling of the material. If this is 
true, then it may be expected that the height of the charge 
may frequently reach almost to the roof. This condition will lead 
to localized heating at the top of the charge; and if there is an open 
vent in the roof such a condition will be magnified for reasons previ- 
ously mentioned. It is, therefore, advisable to have the roof higher, 
as illustrated in Fig. 10, so that even with the maximum charge 
there will be ample space for the gases to become thoroughly equalized 
and to prevent the charge from encroaching upon the hotter zone 
close to the roof. 

A good rule is to make the width and height of door openings as 
small as possible and materially increase the width and height of the 
chamber and thickness of the walls. 

The Heat Reservoir. — Further, when the door is opened in the 
furnace in Fig. 10, there is always a reservoir of heat left in the upper 
part of the furnace to aid in heating the next charge and maintaining 
the temperature of the furnace during discharging or recharging 
operations. With the furnaces in Figs. 5 and 6 this is impossible, 
for when the door is opened the furnace will be almost entirely 
emptied of its hot gases because the door opening is the same height 
as the roof. This, perhaps, more clearly is illustrated by the fur- 

1 This particular example is taken from a well-known spring shop. 



42 



STEEL AND ITS HEAT TREATMENT 



naces in Figs. 11 and 12. It is better practice to have the roof higher 
than the door opening, and always to keep the door height as low 
as possible, as is illustrated by the furnace in Fig. 13. 




Fig. 10. 




Fig. 11. 




Fig. 12. 



Height of Chamber vs. Height of Charge. — In order to make 
quick use of the heat in the furnace, and properly to heat the stock, 
it is a part of the furnace designer's job to provide for ample space 
for circulation throughout the mass. Thus, with the charge arranged 
as in Fig. 11 there is ample room for the circulation of the hot gases 
around the charge, and the heat application would be much better 



HEAT APPLICATION 



43 



and more uniform than in Fig. 12 where the top of the charge is 
near the roof. From this it will be seen ^that the size and arrange- 
ment of the charge is an indication of the heat application value 
of a given shape and size of chamber and type of furnace. 





Fig. 13. 



Fig. 1% 



It is better practice to have the chamber large enough in length, 
width and height to afford plenty of space for circulation, as is illus- 
trated in Fig. 13. This diagram also illustrates the points previously 
made concerning roof vents and door heights. 





Fig. 15. 



Fig. 16. 



Influence of Mass. — The furnaces shown in Figs. 14 to 22, incl. 
illustrate different commercial methods for the pot annealing of 
wire, and are particularly a propos as examples of heat application 
on account of the size of the mass to be heated. 

Thus in Fig. 14, we have a furnace with a combustion chamber 
under the hearth, and with a large pot resting directly upon the 



44 



STEEL AND ITS HEAT TREATMENT 



hearth. No matter how uniform the temperature may be in the 
chamber there will always be a tendency for a cold zone to form at 





Fig. 17. 



Fig. 18. 



the bottom of the pot — as is represented by the shaded portion in the 
drawing. This is due to the fact that the floor of the furnace is of a 





Fig. 19. 



Fig. 20. 



refractory nature, and will not transmit the heat as fast as the pot 
will take it away. The placing of the pots in the furnaces in Figs. 
16, 17 and 18 is better in this respect, inasmuch as it provides for the 
circulation of the hot gases under the pot. 



HEAT APPLICATION 



45 



With a short pot, as in Fig. 14, it is sometimes permissible to 
use a furnace in which the heat is applied from the top of the charge 
downwards; but there is a limit to this because when the height of 
the pot increases, as with Figs. 15, 17 and 18, there will be a ten- 
dency to overheat the top of the pot before the bottom is at the right 
temperature. For this reason the height of the pot (or charge) 
in itself influences the type of furnace. 

In some plants the heat is applied to the bottom (but without a 
separate combustion chamber) and taken off at the top, as in Fig. 




Fig. 21. 



17; but even with this construction it is difficult to secure a uniformly 
heated product, as the temperature of the top and bottom of the 
furnace rarely would be the same. In this respect the design of 
Fig. 18 is better. This latter provides for underfiring, for circula- 
tion under the pot, and the removal of the gases below the hearth. 
With such construction the heat passes from beneath the floor to 
the roof of the chamber and back again to a point below the floor 
level, thus insuring a uniform pressure and temperature of the 
heated gases. 

When the construction in Fig. 17 (with a heavy roof-door) 
takes the form of that in Fig. 19 (with a cast-iron top), as it fre- 



46 



STEEL AND ITS HEAT TREATMENT 



quently does in practice, the method is open to still more criticism. 
In either case the removal of the door, which constitutes the roof 
of the furnace, permits the escape of a maximum amount of heat 
and cools off the furnace. But in Fig. 19 the roof, being of metal, 
radiates a great amount of heat even with the top on the furnace, 
which is severe on the men, wasteful of heat and tends to cool off 
the top of the charge. It is better practice to employ a type of 
furnace in which the charge is introduced through a door or opening 
in the side or end instead of through the top of the furnace whenever 
the size or shape of the charge will so permit. 




Fig. 22. 



Fig. 20 illustrates a construction of pot that is employed very 
successfully in annealing wire and for carburizing; it gives very,, 
good results inasmuch as the heat is applied to the center of the mass, 
as well as the outside, and to the bottom as well as the top. It illus- 
trates the fact that: 

6. The greater the amount of surface exposed to the heat, the greater 
the degree of uniform heating and the less the time required for saturation. 

Figs. 21 and 22 illustrate the relative merits of heating high pots 
from the bottom up (Fig. 21) and from the top down (Fig. 22). In 
the underfired furnace, also provided with a movable ball type 
carriage or charging device, the heat is first applied where it is most 



HEAT APPLICATION 



47 



needed — at the bottom — and the hot gases rising without being 
forced will naturally take care of the heat application at the top. 
In the overtired furnace (Fig. 22) the heat first is applied at the top; 
the gases must descend under pressure and even though the heat is 
forced to circulate under the hearth, that part of the hearth under 
the charge will always be the cold spot, due to the fact that the 
absorptive powers of the iron pot is greater than the heat input 
through the hearth by the hot gases thereunder. The relative advan- 
tages of the two methods of heat application will be more fully dis- 




Fig. 23. 



Fig. 24. 



cussed in subsequent sections, but it is here evident that the height 
of charge again influences the method of applying the heat. 

Influence of Character of the Charge. — A charge in a furnace 
heats up in about the same manner as a plate of ice cream while 




Fig. 25. 



melting — that is, from the top and outside edges towards the center — ■ 
particularly when the heat is applied from above. The difference 
between uniform chamber temperature and uniformly heated product 
is illustrated by Figs. 23 and 24, in which is it assumed that the 
charge consists of small pieces — such as springs or washers — laid 
on the chamber floor. It is immaterial in this case whether the 
furnace is underfired (Fig. 23) or overtired (Fig. 24), or how uniform 
the actual temperature in the chamber may be, because there will 



48 STEEL AND ITS HEAT TREATMENT 

always be a tendency for a cool section in the center, as shown in 
Fig. 25. It is practically impossible to get a uniformly heated 
product under such conditions unless the charge is split up so as to 
permit free circulation of the hot gases through the mass. Unless 
each piece is exposed to the heat in the same manner and under the 
same conditions there is apt to be a variation in the uniformity of 
heating between the pieces. Under such conditions, it is well to 
bear in mind that : 

7. To heat uniformly a number of similar pieces, it is necessary to 
expose the same area of surface of each piece in the same manner, for 
the same length of time, at the same temperature. 

The usual lack of consideration of these points of identical area 
of exposure and time of saturation accounts for the variations that 
so often occur in the finished product without any change in the 
indicated temperature. 

Instead of heating such small pieces, as shown in Figs. 23 and 24, 
it is better practice to employ an automatic type of furnace like 
Figs. 2 and 3. With these designs the entire surface of each piece is 
exposed to the action of the heat, and each piece is heated and cooled 
separately. This also illustrates how the nature and mass of material 
will influence the design of the furnace and why a furnace suitable 
for one operation is not suitable for another, no matter how uniformly 
the chamber itself may be heated. 

Muffle Furnace. — Muffles, as ordinarily constructed for heat- 
treatment work, do not necessarily prevent oxidation, other claims 
to the contrary. The following are statements taken from recent 
publications on this subject: (a) " When oxidation or the formation 
of scale is particularly objectionable, furnaces of the muffle type 
are often used, having a refractory retort in which the steel is placed 
so as to exclude the products of combustion." (b) " The metal 
does not become saturated with any of the products of combus- 
tion . . .", referring to the furnace illustrated in Fig. 30. 

The first statement, referring to the merits of the muffle versus 
the open chamber, must lead to the conclusion that muffles are synon- 
ymous with improper open chamber work. We have previously 
explained that, with proper furnace design and correct operation, 
an atmosphere may be produced which will contain no free oxygen, 
no objectionable fuel vapors, and just enough carbon monoxide to 
take care of any air which may possibly find its way into the heating 
chamber through unforeseen causes; that this slightly " hazy " or 
reducing atmosphere results from an absolute control of the air and 



HEAT APPLICATION 



49 



fuel supply, in combination with the right furnace design; and that 
a furnace operated in such a manner will give a product which will 
often be better than that heated under charcoal. Thus, when it is 
found necessary to exclude the products of combustion from contact 





Fig. 26. 



Fig. 27. 



with the hot steel, it means that such gases contain free oxygen, and 
which is idential with improper furnace operation or design. 

Again, if the products of combustion are excluded from the 
muffle, the question reverts to the fact that there is no method of 





Fig. 28. 



Fig. 29. 



keeping the outside air or oxygen from finding its way into the muffle. 
As there is no pressure of gases from within, since the pressure which 
should be caused by the products of combustion is absent, the free 
oxygen must inevitably find its way into the muffle. For these 
reasons, therefore, it is desirable to employ some means of destroy- 
ing the oxygen within the muffle: This usually is accomplished with 
charcoal or some similar material. Muffles, 1 however, are desirable 



50 



STEEL AND ITS HEAT TREATMENT 



in connection with certain delicate operations, and whenever the 
design of the furnace or the method of operating is not likely to 
produce and maintain the proper heating conditions or atmosphere 
within the chamber, such as in enameling. 

Semi-Muffle Furnaces. — On the other hand, if a pressure of gases 
from combustion is built up within the muffle to prevent air from 
entering, there must be some opening between the muffle and the hot 
gas chamber surrounding it. In such a case it is no longer a true 
muffle and does not exclude the products of combustion. Thus, in 
Fig. 30 there are shown openings in the roof of the muffle. If the 
course of the hot gases is such that the gases go downwards through 
these openings, then why have any roof at all on the muffle? — for the 




Fig. 30. 



products of combustion enter the heating chamber and the furnace 
approximates open chamber construction. Or, if the flow is upwards 
through these openings, then outside air will be sucked into and 
through the muffle, and oxidation will be set up in that manner. In 
any event, the absence of flues leading from the chamber makes it 
necessary to force the gases in under pressure, because the natural 
tendency is to short circuit directly to the outlet. In other words, 
whether or not the furnace, as, and for the purpose designed, is 
properly operated, there is no occasion for the remaining solid part 
of the muffle roof, and the statement contradicts itself. 

Influence of Nature of Fuel on Furnace Design. — The overtired, 
perforated-arch type of furnace — under which Fig. 30, previously 
discussed, may be classed, and which general type is common to the 
construction shown in Fig. 31 — is a development originally intended 
to meet certain conditions with oil fuel, but which is neither necessary 



HEAT APPLICATION 



51 



nor desirable with gas or coal. The development also illustrates the 
difference between uniform application of heat and uniform applica- 
tion of fuel. 

With a type of rotary annealing furnace like that illustrated in 
Figs. 28 and 29, Using gas as a fuel, the burners are usually placed 
on both sides, so that a small amount of fuel is injected through each 
burner. This distributes the application of the fuel and heat. With 
oil, however, the consumption under a similar arrangement of burners 
would be so low that the burners could not be kept going steadily, 




Fig. 31. 



and for this reason it is desirable to employ but one burner. This, 
in turn, is open to the objection that there would be a hot streak 
directly in front of the burner and which would react unfavorably 
on the charge. To overcome this the perforated arch is employed 
so as to lessen the streaking effect of the flame and thus distribute 
the heat, as is shown in Figs. 26 and 27. In this manner there is 
secured a uniform distribution of heat even with a local application 
of fuel. 

It is therefore evident that the nature of the fuel in these cases 
determines the design of the furnace. In the first case (Figs. 28 and 
29) the uniformity of heating, aside from the nature of the charge, is 



52 



STEEL AND ITS HEAT TREATMENT 



secured by a uniform burning of the 
fuel — gas — throughout the length of 
the furnace. In the second case (Figs. 
26 and 27) the fuel — oil — input is con- 
centrated and the perforated arch con- 
struction is employed to secure the 
heat distribution. The perforated arch, 
which was found advisable in the case 
of oil, for the particular purpose in 
view, is entirely unnecessary with gas 
fuel. 

Another illustration of this perfor- 
ated-arch type as applied to a long, 
narrow, low hearth with concentrated 




Fig. 32. 

fuel supply, and which followed the 
above development is shown in Figs. 
32 and 33. In this case also the fuel 
input is very low, due to the com- 
paratively light stock heated in the 
chamber; the perforated arch distrib- 
utes the heat to the chamber beneath 
and from which the gases pass under- 
neath the hearth. It should be borne 
in mind that while the perforated arch 
construction may be perfectly proper 
under certain conditions, as above 
noted, it may be entirely out of place 
under other conditions, and yet using 
the same fuel. 



HEAT APPLICATION 



53 



Perforated-arch Furnaces. — From the type of furnace of Figs. 
32 and 33 it is but a short step to the overtired, perforated-arch 
furnace shown in Fig. 31 (previously alluded to), and which has 
been somewhat widely employed for general heat treatment work — 
often regardless of distinctive shop and heat application conditions. 
It will be noted that there are burners on both sides of the furnace 
(generally staggered), that the combustion takes place in a cham- 
ber above the main working chamber, and that the gases then pass 
down through the perforated arch into that chamber and are taken 
out from under the floor. 




Fig. 34. — De-carburization of Steel by High- velocity Gases. X60. (Bullens.) 

This design provides, in effect, for the application of heat from 
above through a perforated arch, and is permissible with low charges, 
but when the charges are high there is a natural tendency to over- 
heat the top. If a thorough study is made of the perforated arch 
itself, it will be found that the actual openings only total about 
25 or 30 per cent, of the total chamber area. With a continual 
input of fuel and air into the hot combustion chamber above, and 
with but a small exit for the hot gases, these hot gases must enter 
the working chamber at a comparatively high velocity. If the fur- 
nace is charged to anywhere near the hsight of the working opening 
or arch, which is a common procedure, the hot gases will impinge 



54 STEEL AND ITS HEAT TREATMENT 

first upon the top of the charge, and at high velocity if the fires are 
forced. This inevitably results in severe cutting action and oxidation, 
the zone of which is shown in the charge in Fig. 31. This is further 
illustrated by the photomicrograph of Fig. 34, taken from the edge 
of an annealing charge of chrome nickel steel plates piled as illus- 
trated in Fig. 31. It will be seen that the steel has been entirely 
decarburized along one edge, even though the actual indicated tem- 
perature of the furnace was only about 1350° F. In the plant from 
which this example was taken it was no uncommon occurrence to 
lose as much as \ or even \ in. of metal on each side of a pile 10 or 12 
in. wide. 

Aside from changing the type design of the furnace, the only 
method of overcoming this particular trouble without decreasing 
the production is to increase the height of the working chamber. 
In this manner the gases are given an opportunity to expand before 
reaching the metal, and thus reduce the high velocity caused by the 
perforated arch. The overtired, perforated-arch type of furnace is 
permissible with low charges in comparison with chamber height, 
such as is shown by the dotted fine charge in Fig. 31, as the ten- 
dency to overheat at the top has been shown on low charges less than 
1 ft. in height, just the same as in charges 4 ft. in height. The 
height of the charge itself is immaterial, as it is the relationship 
between the top of the charge and the under side of the perforated 
arch that is the critical point. 

Overtired Furnaces. — In order to overcome the necessity for 
comparatively high-working chambers, as occasioned by the con- 
ditions above referred to, the perforated-arch construction may be 
eliminated. This results in a type of overtired furnace illustrated 
in Fig. 35. In this case, as in the other, the charge is heated from the 
top downwards, and the gases pa,ss out from under the floor. Con- 
siderable space is provided above the charge to afford an opportunity 
for the gases to mix and build up a natural pressure throughout the 
unrestricted area of the chamber. Aside from the fact that the hot 
gases have to pass downwards, the question then arises as to the 
manner in which the charge is heated. Since the heat is applied 
from above there is no question but that the top of the charge will be 
heated; but how about the bottom of the charge? 

It should be borne in mind that this construction — Figs. 31-35 — 
(with flues under the hearth) does not necessarily result in a hot 
floor, even though it be granted, for sake of argument, that there is a 
considerable volume of gases under the floor and that the tempera- 



HEAT APPLICATION 



55 



ture of these gases is the same as above the floor. The specific heat 
of the charge is such that it absorbs heat from the floor, and unless 
the rate of input through the floor is greater than the rate of absorp- 
tion the floor will cool under the charge in proportion to the manner 
in which it is packed. This cold spot, in turn, means a cold zone 
through the center and bottom of the charge, and until this cold zone 
is removed the charge is not uniformly heated. But in order to 
remove it, it is necessary to lengthen the time of exposure, which 
results in a tendency to expose the outside edges of the charge to 
the action of the heat and gases longer than is necessary with other 
construction. In practice this cold spot is never totally eliminated 
in this type of furnace. 

Even with the I-bar floor construction illustrated in Fig. 35, 
which is the best in use for this type of furnace, the floor is not as 
hot as it should be. The reason for this is that, irrespective of 
the temperature or volume of heat under the floor, the rate of trans- 
mission of heat to the under side of the charge is no greater than 
that possible through the vertical section of the I-bar. The rate 
of transmission through the tiles separating the bars is still less than 




Fig. 35. 



through the bars themselves on account of the low conductivity 
of the material. When the construction is made without the 
I-bars, as it sometimes is to lower the cost, the conditions are still 
worse. 

From this it will be seen, as has been repeatedly proven in 



56 STEEL AND ITS HEAT TREATMENT 

practice, that there is a disadvantage in any construction which 
does make possible a floor temperature equal to that above the 
charge; or a circulation of heat under the charge to make up the loss 
in transmission and decrease the time of exposure by decreasing 
the area of the cold zone. 

Influence of Arrangement of Charge. — Such a condition just 
described illustrates very forcibly the difference in heat application 
obtained when a furnace is full and when it is empty. That a 
furnace will give uniform temperatures without a charge is no cri- 
terion that the heat application to a charge will be uniform. The 
type of furnace illustrated in Figs. 36 to 41 is used extensively in 
the annealing or wire and tool steel, and many people wonder why 
it is that with almost perfect pyrometer records they do not get a 
uniform product. 

The furnace is fired with coal from a fire-box at one end, the flame 
and heat passing over a bridge wall to the heating chamber; at the 
other end of the hearth the hot gases pass down and under the 
hearth through a series of flues, and thence to the chimney. The 
hottest part of the hearth is near the bridge wall. 

In the case of the charge of wire in Figs. 36 and 37 the non- 
uniformity of product is partly due to the fact that the first piece 
in is the last piece out and vice versa, so that the first piece is exposed 
to the highest temperature for the longest time, and the last piece 
to the lowest temperature for the shortest time. 

With the method of charging tool steel in Figs. 38 and 39 the 
non-uniformity is partly due to the lack of circulation through the 
charge; the tubes rest directly upon the hearth and are packed 
tightly together. This can be somewhat overcome by rearranging 
the tubes as in Figs. 40 and 41, separating them and raising them 
up from the hearth. 

All of these (Figs. 36-41) are open to the objection that the 
heat is not uniform throughout the length of the charge, and while 
it is possible to vary the quality of the product by rearranging the 
charge without affecting the pyrometer readings, as above illus- 
trated, there is still the fact that the heat should be uniformly applied 
throughout the entire length and through the mass in order to secure 
a uniformly heated product. 

This latter point is also illustrated by Figs. 42 and 43. In 
carburizing work the practice of Fig. 42 is often followed, rilling the 
furnace to its maximum capacity by packing the boxes up to the side 
walls as well as in front. It is much better practice to maintain 



HEAT APPLICATION 



57 




58 



STEEL AND ITS HEAT TREATMENT 



circulation space on the sides and ends, as shown by Fig. 43, and 
not to place the boxes beyond a certain imaginary line such as is 
illustrated by the dotted line of the drawing. The method of hand- 



f//// 


W//////////////////A 


i 










I 


i 







W//////////W//////A 


"'//t/ 


« 










i 







Fig. 42. 



Fig. 43. 




Fig. 44. 



ling or arranging the charge in the furnace is equally important 

with correct furnace design and proper operation or heat distribution. 

Other Furnace Designs. — The designs of furnaces intermediate 

between the two principal .types — underfired and overfired — are 



HEAT APPLICATION 



59 



legion. One characteristic furnace in common use is that illustrated 
by Fig. 44. In this it will be noted that the underfiring principle 
has been used, locating the combustion chamber under the hearth; 
that the hot gases pass upwards to the heating chamber on one 
side of the hearth; and that a roof vent is located on the opposite 
side of the chamber. In this design the benefit of underfiring is 
largely negatived by the poor heat application to that part of the 
charge and hearth directly under the vent and farthest removed 
from the heating chamber intake ports; the hot gas currents will 
short-circuit from the ports to the vent. A good overfired furnace 
would be better practice. 

Coal Furnaces. — Fig. 45 illustrates a common type of heat treat- 
ment furnace using hard coal as the fuel. The heat is generated 
from coal placed on the grate at the left of the furnace illustrated, 
passes over the bridge wall into the heating chamber, and then under 
the hearth to the flues and to the chimney. From points previously 
raised upon other furnaces there will be noted the tendency to 
localized heating near the bridge wall, the fact that the height of 




Fig. 45. 

the door opening is virtually the same as that of the roof arch, 
and the tendency towards a cold hearth. In such coal furnaces as 
generally designed and operated there is a decided lack of control 
of the volume, temperature and composition of the gases to and from 
all points in the chamber. 



60 



STEEL AND ITS HEAT TREATMENT 



Again, when local conditions advocate the use of a cheap fuel, 
the general method of burning it, as here illustrated, is found to be 
inefficient from the standpoint of fuel application and unsatisfactory 




Fig. 47. 



from the heat application viewpoint. The only proper way to attack 
such a problem is first to gasify the coal and properly to utilize that 
gas; not necessarily to generate the gas in a separate producer 
and carry it by expensive flues to the furnace, but to combine the 
two operations in one efficient, self-contained unit. Such is being 



HEAT APPLICATION 



61 



done, and such furnaces are today producing a better heated product, 
at less operating cost, than many furnaces using oil or gas. 

Car Furnaces. — Figs. 46, 47 and 48 represent three designs of 
the car type furnace — the open-chamber, perforated-arch and coal- 
fired furnaces respectively. 

Furnaces with the movable car-bottom may be mechanically 
efficient, but they are thermally inefficient. In cases where there is 
much work to be handled of a large and variable shape and size, such 
as long and heavy crank-shafts, irregular-shaped castings and forg- 
ings, etc., the use of such furnaces may be permissible from the 
standpoint of handling. It is the same proposition of heating from 




Fig. 48. 



above and cold hearths versus hot hearths which previously has been 
discussed, but it carries the matter one step farther. 

In this case, each time the hot car is removed from the furnace a 
large amount of hot chamber gases is lost, and the furnace must be 
retired with a cold hearth and comparatively cool walls and roof. 
Similarly, the radiation losses during the time between the removal 
of one charge and the placement of another are very great. This 
point is evident when it is considered that the actual area uncovered 
for the removal of a charge extends from the top of the opening 
in the floor on which the car runs, as is indicated by the lines a and b 
in Figs. 47 and 48. Even though the door is lowered directly after 
the car is withdrawn, there still is uncovered the area represented 
by the width of the opening and the height from the floor to the top 
of the car. This condition is not so pronounced in double-end fur- 



62 STEEL AND ITS HEAT TREATMENT 

naces so arranged that one car is drawn into the chamber as another 
is drawn out, though there does exist the disadvantage brought about 
by the necessity for uncovering the opening at each end of the 
chamber at the same time. In any event, the heat absorbed by the 
top of the car while in the furnace is practically lost when the car is 
withdrawn and must be replaced at the expense of time and fuel 
when the car is recharged. 

Any advantage that this type of furnace may have for annealing 
under the conditions noted are minimized in connection with a heat- 
treatment process requiring quenching after heating. Under such 
conditions it not only is necessary to heat each piece uniformly, but 
also to cool it uniformly. Even though the pieces come out of the 
furnace heated alike, it is difficult to quench them all at the same 
temperature. The charge on the car naturally cools rapidly and it 
usually is found that the interval of time between the quenching of 
the first and last pieces results in the cooling of the pieces from dif- 
ferent temperatures, thus tending to defeat the real purpose of the 
process. For these reasons it is evident that furnaces with car 
bottoms are entitled to consideration only when the handling of the 
material determines the point, and that they should not be consid- 
ered whenever a type better from the standpoint of heat application 
can be employed without unreasonable difficulty in handling the 
material in and out of the furnace chamber. 

Underfired Furnaces. — The aim of any heating operation should 
be to supply heat uniformly to all surfaces of the charge. Under 
ordinary manufacturing conditions this is accomplished most nearly 
in the underfired type of furnace with a perforated floor, and with 
which an effort has been made to overcome the disadvantages pre- 
viously referred to in the discussion of other types of furnaces. 
Fig. 49 illustrates a recently patented type of this underfired furnace: 
This, however, is intended to use with the heating of material handled 
on pans and is not applicable to the general run of heat-treated work. 

The heating is done from the bottom upwards instead of from 
the top downwards. Heat naturally rises, and with such construc- 
tion as in Fig. 49, if the floor is hot the roof is hot, although it is 
possible to obtain the reverse in an overtired furnace. Even in an 
underfired furnace the bottom can never be heated more than the 
top. In the construction outlined in the drawing, the gases are 
passed through large combustion chambers and compelled to circulate 
through several hundred feet of ports on both sides, as well as through 
the floor, which exposes considerable area against which the gases 



HEAT APPLICATION 



63 



are wiped. In this way there is a minute subdivision of the volume 
and a thorough mixture. 




Fig. 49. 

Such a furnace design is also in accord with the fact that correct 
heating is a function of pressure. Thus the heat, in going to the 
roof, naturally stratifies and builds up a natural pressure, which 
will be spread over the entire area before it will come down. The 
result is a pressure always on the floor and the elimination of streams 
of gases of unequal temperature and composition. The hot gases 
surround the steel as a blanket, and have a minimum velocity. 
With such methods of applying the heat, the area of the cold zone is 
quickly decreased, and this, in turn, lessens the length of exposure. 

It will also be noted that provision is made for circulation on 
both sides of the charge, independent of the manner in which it is 
packed. It is impossible to overload the furnace and cut off the 
circulation, and even though the charge were the full width and 
height of the working opening there would still be room on each side 
for circulation. In the particular furnace illustrated, this extra 
room costs about 3 feet in chamber width; the area for circu- 
lation on the sides is about 30 per cent, of the width of the door, 
and it is by the room afforded with such greater width that the 
velocity of the gases is cut down. 



64 STEEL AND ITS HEAT TREATMENT 

On the other hand, as with the furnaces illustrated by Figs. 
31 and 35, it will be noted that if the chamber is the same width as 
the working opening, it will be necessary to employ comparatively 
small charges in order to get circulation through the restricted 
areas on the sides. To gain time, with this practice there is a danger 
of overheating for the reason that, as the area is decreased, the 
pressure must be increased for a given B.T.U. input, which works 
out in practice, as a rule, to the detriment of the top and exposed 
edges of the charge. 

Flue Construction. — For furnaces of any considerable size, flue 
construction is absolutely necessary, not only to provide an escape 
for the waste gases, but also to direct the hot gas currents during 
their passage through the furnace, and to conserve the heat in those 
gases after they have left the heating chamber proper. Primarily, 
the practice is to circulate the gases around the stock to be heated, 
to heat the chamber as a whole, and then to pass them out at the 
coldest part of the chamber. Some of the furnace drawings given 
have shown in some degree such provisions, but in order not 
to complicate the discussion of heat application, the subject of 
heat conservation has been little dwelt upon. The latter is, in 
fact, a problem which must be studied out for each particular 
design. In any case the flues should be arranged so as to pre- 
vent short-circuiting of the hot gas. cycle, and give up as much 
heat as possible to the furnace structure before passing to the 
atmosphere. 

Conservation of Heat. — Proper flue construction and thicker 
walls both tend towards the conservation of waste heat. The dis- 
cussion thus far has had to do with single furnaces, and the extent 
to which the thickness of the walls might be increased is obviously 
restricted within narrow limits by the cost of construction. Since 
losses by radiation are largely preventive, any arrangement or group- 
ing together of furnaces of a similar type which will tend to unite 
them and decrease the radiating area, thus eliminating exposed walls, 
should be made a subject of study. 

Variety of Furnace Plans. — The diagrams in Figs., 50, 51 and 52 
are intended to illustrate as floor plans a few of the manjr different 
furnace designs which are employed in practice. All of these, as 
well as hundreds of others not shown, have been built in a variety of 
sizes for oil, gas, coal, coke and wood, with different methods of 
applying the heat to suit different operations ranging from small 
needles to eighty tons of steel at a charge. 



HEAT APPLICATION 



65 



In designing furnace equipment it is not only necessary to con- 
sider combustion and the more important points of heat applica- 



^ 


m^$$^ 






!_ 


^$$^ 


^ 




-;; 


^^^> 


*$ 




tion to the stock as well as the fuel suited to both, but likewise the 
method of handling material to and from the furnace, together with 



66 



STEEL AND ITS HEAT TREATMENT 



the floor space available, which are no small factors in the cost of 
production and installation. 

The purpose should be to keep the material, the men, the fur- 
naces and machinery in continuous operation, or as near it as possi- 
ble, because each is more or less dependent upon the others and 
all must be properly linked together to secure the best all-around 
results. It is just as necessary to adapt the furnace design to manu- 
facturing conditions as it is with machine tools, but the latitude for 
variation is much greater. 





m w. 






Fig. 51. — Unit Furnace System Development. 



Owing to the great variety of heating operations and shop 
conditions, it is rarely found that the same identical furnace can be 
properly employed in two shops, or even in separate departments 
of the same shop, for similar operations. 

These sketches will serve to illustrate some of the development 
that has been made, as well as the latitude possible in designing 
furnace equipment, and further shows that furnaces cannot properly 
be standardized with regard to the number, shape or sizes of the 
chambers or the method of applying heat to the charge owing to the 
great variety of conditions which must be met. 



HEAT APPLICATION 



67 



Unit Furnace System. — Every large factory employing heat 
treatment methods is more or less confined to a general type of 
product which requires some particular and standardized treat- 
ment. Such conditions would seem to be naturally and readily 
adapted to the small-furnace unit practice, having elasticity of pro- 
duction as its general aim. Thus it is no unusual sight to see ten or 
fifteen small, light furnaces, usually on legs, of similar type and size, 
strung out in a row. And yet if the manager of that plant were 
to visit a boiler room where each of ten or more boilers were set 
off by itself, how caustic would be his comments! The same prin- 
ciples of heat generation, utilization and conservation are applicable 
to both. Not only does the unit system of furnace arrangement 
require, as a general rule, more care and attention, involve more 




u 

; Y777777A 1 




Fig. 52. 



steps for the furnace man, longer time for heating and general inef- 
ficient handling, but it also tends to raise the cost of operation and 
maintenance out of all proportion to the product turned out. Such 
an arrangement wastes floor space and increases the radiating area 
which, with the usually light walls and uncontrolled discharge of 
gases at the maximum (instead of the minimum) chamber tempera- 
ture, greatly increases the temperature of the room and creates a 
very uncomfortable and unhealthf ul working condition for the men, 
to say nothing of the greater cost for fuel, power and maintenance. 

Experience has shown that three, or perhaps four, small single 
furnaces of the same type for the same work, are about the general 
economic limit of the small-furnace unit system. 

To explain some of the diagrams in Figs. 50, 51 and 52, previously 
referred to, and to develop the growth of the multiple furnace, we 
might commence with a, b and c of Fig. 50. 

In (a) the door opening is the full width of the chamber, giving 
opportunity for packing the charge to the full width of the chamber: 



68 STEEL AND ITS HEAT TREATMENT 

such a condition, as previously explained, is usually not advisable, 
as it tends to cut off circulation. It may be remedied as in (a) by 
placing jambs on each side of the front. 

In (c) there is an opening at each end of the furnace, allowing 
for charging at one end and discharging at the other, or for working 
the same operation from both ends simultaneously. Such construc- 
tion is thermally bad in that it permits a direct draft from one door 
to the other, with consequent loss of heat. If it were a question of 
doing the same work from each side at the same time, the layout might 
be changed as in (d) or (e) to prevent the cold air draft. Or if it were a 
matter of charging and discharging simultaneously without interference, 
the design might better take the form of (/), having the two openings 
at one end, and widening the furnace to make up for loss in length. 
In the latter construction one-half of the chamber could be unloaded 
at a time and one or both sides recharged at the same time. 

Twin Chambers. — The next logical step in avoiding the small-unit 
system is to use twin-chamber construction. For each exposed 
wall of the unit system which is removed, the more can we increase 
the sturdiness of construction, and there diminish the heat radia- 
tion loss of the remaining walls without additional construction cost. 
Thus the development of two furnaces of the (a), (6), (c) or (d) 
type, Fig. 50, will be that as shown in (aa), (bb), (cc) and (dd) 
Such furnaces will have the advantages of better heat application 
and conservation, and perhaps of handling material, but will have 
the disadvantage that if one side " goes down," the other will usually 
have to go out of commission also in order to effect repairs. 

In some cases the dividing wall may be omitted as in (g) and (h ) , 
giving one large hearth. But in this case only one temperature 
work can be done at one time in place of the two temperatures pos- 
sible in the twin-chamber construction. 

Furnace Batteries. — There are many plants which use small, 
light furnaces, built on legs which might advantageously combine 
the smaller units into heavy batteries and obtain both better heat 
application and handling methods. Other things being equal, the 
construction and arrangement of Fig. 51 (6) and (c) would be much 
better than that of (a). 

Other designs as necessitated by distinctive conditions of material 
handling or shop efficiency are illustrated in Fig. 52, and may be 
varied, of course, ad libitum. 

General Furnace Data. — Full data should be gathered as to the 
sizes, shapes and approximate output, both maximum and minimum, 



HEAT APPLICATION 69 

of the material to be handled, besides the specific treatment desired; 
also the floor space available and the most convenient method of 
handling material to and from the furnace room and the furnace 
units in the room. 

Careful consideration then should be given as to whether or not the 
operations can be made mechanical, i.e., automatic or semi-automatic, 
or practically continuous without automatic equipment. From this 
information the specific type and dimensions of the chambers can be 
deduced. As a general principle, it may be stated that the heating 
chamber should be large, but with a minimum area of door opening. 
The furnace should be of the best possible design to suit the par- 
ticular work in hand and the existing factory conditions. The spe- 
cific purpose for which the furnace is to be used should be decided 
definitely — whether for annealing, hardening, toughening, carbur- 
izing, etc., or for a combination of such operations. In connection 
with this it may be said that the maximum efficiency of any heat- 
treatment operation or furnace is obtained by using a furnace espe- 
cially designed for that purpose and operating continuously on one 
class of work at one temperature. 

Specific consideration should be the quality and cost of the finished 
product. The seven rules given on pages 32, 33, 46 and 48 should 
govern the selection of the type of furnace and the fuel to be employed. 
The plant conditions governing the amount of material to be heated, 
methods of routing and handling, floor space, building location, light, 
ventilation and working conditions for the men, source and kind of 
power, hours of working, standards of quality, class of men, etc., 
should determine the number of furnaces, arrangement and sizes of 
chambers and openings, type and position of motors, blowers, pumps, 
piping and other auxiliary apparatus. 

The entire proposition should be considered and determined as a 
complete manufacturing unit, designed to be made a part of, and 
adapted to, a general manufacturing process, and to suit the indi- 
vidual requirements and conditions of the particular plant for which 
it is intended. 

It is only a proper combination of all these essential and deter- 
mining features, coupled with the human element represented in the 
operations, that will make possible the conduct of a good heat-treat- 
ment process and lay the foundation for improvements in methods 
and equipment for their efficient application. 

The Practical Solution of the Heating Problem.— The practical 
and only really effective solution of the heating problem, in so far 



70 STEEL AND ITS HEAT TREATMENT 

as it relates to heat-treatment work, is covered by the following 
rules : 

I. Select the furnace equipment best adapted to the production 
of the results sought for, from the standpoint of heat application and 
uniformly heated product and in harmony with the manufacturing 
requirements and plant conditions. 

II. Select that furnace equipment which not only meets the 
requirements of (I) , but, in ' addition, is adapted to the heating 
requirements and plant conditions from the standpoint of heat 
utilization or conservation of the heat generated, which means that 
the furnace design should include: 

(a) Proper provision for thorough combustion of the fuel and with 
complete control of all air and fuel entering the furnace; a uniform 
delivery of heat to the working section of the chamber without 
risk of unevenly heating the charge in the chamber; 

(6) Proper flue arrangement relative to the delivery of heat to 
the charge in the chamber and removal cf the gases from the chamber; 

(c) Proper arrangement of flues for a prolonged passage of gases 
from the chamber and through the furnace structure before discharge 
from the furnace; suitable arrangement of flues and dampers for 
the control of gases as discharged from the furnace; 

(d) Substantial construction for: Stability; retention of heat 
when the fires are off and the dampers closed; decrease of heat losses 
through radiation, sudden cooling off, etc.; 

(e) Proper mechanical arrangements for: Convenient and safe 
control of fires, dampers, doors, valves, etc.; handling of material 
to be heated; convenience of repairs; comfort and safety of the men; 

(/) Insulation and means for utilizing, whenever practicable, the 
heat in waste gases. 

III. Select equipment with such consideration of the number, 
size, shape of chambers and working openings as will provide for a 
minimum number of furnace units in order to conserve fuel by de- 
creasing the surface exposed for radiation from walls and from an 
unnecessary number or size of working openings. 

IV. Select that fuel which is best adapted to the heating require- 
ments and plant conditions previously determined and which, 
with proper operation of the best furnace equipment is likely to 
cost the least from the standpoint of finished product, regardless of 
its cost on a heat unit basis. 

V. Provide intelligent furnace operators and active supervision 
over the heating operations in order that the results made possible 



HEAT APPLICATION 71 

by the design and layout of the equipment will be produced with a 
minimum waste of fuel, time, power and the direct and indirect 
waste of money and business reputation that follows in the wake of 
improperly heat-treated product. 

Practical Notes. — In concluding the discussion of " heat appli- 
cation " thoughtful consideration of the following additional sug- 
gestions on heating is asked: 

1. It is the uniformity with which the heat is applied to, and 
absorbed by, the steel that is the real test in heating. 

2. Indication of uniform temperature in a furnace does not prove 
the existence of a uniformly heated product within that chamber. 

3. Uniformly heated product involves a consideration of tempera- 
ture, time, mass and surface, and not of temperature alone. 

4. Uniformly heat-treated product involves a consideration of tem- 
perature, time, mass and surface, not only in heating, but also in 
cooling. 

5. The pyrometer is merely a gauge to indicate the existence of 
heat energy or heat potential, which is but one of the several elements 
affecting the results. 

6. It is necessary to watch the pyrometer, the clock, the area of sur- 
face exposed to the heat and the character of the "heat " or atmosphere 
in the chamber in order properly to control the heating operation. 

7. Combustion of fuel and generation of heat are incidental 
to the actual operation of a furnace. The final result hinges upon 
the design and operation of the equipment utilizing that heat. 

8. Fuel cost is not heating cost. One is at the start and the other 
at the finish of the operation. It requires good equipment and 
operation to lower the latter, just as it requires a good car and a 
good operator to lower the cost per ton-mile with a motor truck. 

9. The results in heating are not as easily determined, controlled 
or checked as are results in machining, rolling, stamping, etc., on 
account of the fact that the selection of equipment and men for the 
operation is a matter requiring a very careful consideration of the 
facts, and that the best is none too good. 

10. It is the proper combination of the right furnace and the 
right man — like the stove and the cook--that determines the result; 
and, in one case as in the other, the judgment of the man is of vital 
importance. 

11. The average run of furnace equipment and furnace opera- 
tion is below the standard necessary properly to meet modern require- 
ments. If a good metallurgist and laboratory equipment are neces- 



72 STEEL AND ITS HEAT TREATMENT 

sary to determine " how to do it " on a small scale, it is necessary 
to have good operators and equipment to actually do it on a large 
scale. One determines the technical method: The other determines 
the commercial value of that method. 

12. The selection of equipment and fuel for heating or heat 
treatment, as for power, is primarily an engineering problem based 
upon an analysis of the individual plant conditions, and not merely 
one of buying or selling. The field is too broad, the manufacturing 
requirements and plant conditions too variable, the room for im- 
provement too great, and the possibilities too little known generally, 
to permit of proper standardizing or of selecting in a manner pos- 
sible with equipment or stock which can be standardized or catalogued. 

13. There is no one fuel, method of burning, " efficient burner," 
shop layout, method of heating or handling, pyrometer, or any " one " 
thing which will in itself partly satisfy every condition, or even one 
condition completely, regardless of what ideas may be found to 
the contrary through the influence of " talking points " or other sug- 
gestions based mainly upon commercial considerations. 

14. The problem of heat treatment begins with the casting of 
the ingot, and ends with the finished product after the last cooling 
operation. 



CHAPTER IV 
THE HUMAN ELEMENT 

Importance of the Human Element. — The importance of the 
human element in heat-treatment operations cannot be too highly 
estimated, even though the average present-day practice in many of 
the largest shops in the country indicates that it is not considered an 
important factor. The man is the keynote to the situation and, at 
the same time, the weakest link in the chain. He is the cook or chef 
that puts the metallurgical finish on the material prepared at great 
expense by others, and it is his skill and exercise of judgment that 
determine the final result and either make or break the cycle of 
operation. 

The present-day cycle is something like this: Skilled metallur- 
gists and mechanics, with expensive plants and equipment, are 
employed to make a steel of suitable chemical composition. Then 
there are employed skilled engineers, draftsmen and mechanics 
with more expensive equipment, to shape and form it. The calcu- 
lations of these men are based on the research of the metallurgists, 
who indicate the results likely to be expected from the steel with 
proper heat treatment. When all this labor and expense have been 
put into the manufacture and elaboration of the steel and it is ready 
for the metallurgical finishing touches, does it go to other skilled 
artisans, who, with proper equipment and knowledge of the results 
sought for, complete the final and all important operation? No. 
As a rule, and which investigation shows to hold good in a large 
number of leading plants in the country, the work is entrusted to 
men who, as a matter of hard, cold fact, do not understand heating 
and whose knowledge seems to be limited to the extent of burning 
fuel, making a fire and watching a pyrometer. There is a mechan- 
ical check of some kind upon everything but the man that controls 
the heat-treatment operation. 

" But," the average manufacturer will say, " I have the best 
pyrometer system that money can buy. I have instruments that 
automatically record in the shop and in my office the temperature 
variation in each furnace, so that I know just what each man is. 

73 



74 STEEL AND ITS HEAT TREATMENT 

doing. In addition, I have men whose sole duty it is to watch these 
instruments and to indicate to the operator by a red, green, or blue 
light just when his temperature is right and when it is wrong. I 
have P. and X. oil burners, which are the best made. My chemist 
analyzes the steel when it arrives at my plant and says it is all right. 
I have the same type of furnace as A. and B., who have been heat 
treating in their own shop for years and who ought to know what to 
recommend. Their head hardener, who has had years of experience, 
designed these furnaces and ' Old Bill,' who has been building fur- 
naces for years, put them up for us. I have the best there is, and if 
anything's wrong with my steel when it's finished I know it's in the 
steel and not in the heat treatment." 

Such is about the view of the average manufacturer, who reasons 
along these lines and thinks he knows what good heat treatment is. 
He is sincere in his belief and thinks that no one has anything better 
than he when it comes to being " up to date." 

In the final analysis, this is nothing but reasoning by analogy and 
is not necessarily logical nor conclusive. It all hinges on the men 
that have had the " experience " and who are supposed to know, or, 
at least, ought to know, but, in the majority of cases it will be found 
that these same men have learned their lessons in one plant and are 
inclined to follow the methods of that plant, and have not sufficient 
acquaintance with modern general practice to express an opinion 
that can be accepted as authoritative or conclusive. There is no 
scientific nor technical analysis of the question as with a manufac- 
turing process. It is all based upon the reasoning, or, perhaps, the 
prejudice or mere offhand opinion of men, whose main qualification 
appears to be that they have been doing the work for a long time, 
and, therefore, are presumed to know what they are talking about. 
At best, this is but a weak foundation upon which to base such 
important work if the methods, equipment, and real ability of the 
men in the majority of shops are, in so far as a knowledge of heating 
is concerned, the measuring standards. 

The Human Element and Basic Heat-treatment Conditions. — 
What is it that determines when the charge in the chamber is thor- 
oughly saturated with heat to the temperature indicated by the 
pyrometer? 

What determines the manner in which the charge is placed in 
the furnace and the room for circulation throughout the mass? 

What determines when the bottom and center of the mass are at 
the same temperature as the top and outside? 



THE HUMAN ELEMENT 75 

What regulates the flow and composition of gases in the chamber 
around the stock and the discharge of heat from the chamber? 

What determines if each piece is heated like every other piece 
and is uniform throughout, and whether each piece goes into the 
quenching bath at the same temperature as all others and at the 
temperature indicated by the pyrometer? 

What is it that controls the flow of air into the furnace or the 
flow of gases from the furnace, and by so doing determines whether 
the atmosphere surrounding the stock is oxidizing or neutral, and 
whether the fuel is conserved or wasted? 

These are some of the elements that affect proper heat treatment 
and they are determined not by a pyrometer or furnace or similar 
apparatus, nor by any mechanical means, but by the same methods 
that govern the quality of the products of the kitchen, the judgment 
and skill of the operator. 

The Effect of the Human Element on Improvements. — Some of 
the best moves in the direction of effecting improvement in heating 
methods and the design of furnaces for the purpose have been seri- 
ously hindered and practically condemned by furnace operators who, 
through ignorance, prejudice or for other reasons have maintained 
that the design or operation was not what it should be. Some of 
this is due to the natural inclination of most men to resist a change 
and to illogical reasoning of the cause of the effect. In one promi- 
nent case, an improved design of furnace was developed which 
required a different method of handling than that to which the men 
had been accustomed. The design was the result of slow, empirical 
development, and every feature of it had been proved in practice 
before being embodied in this particular design. Samples of the 
gases surrounding the stock indicated a practically perfect atmos- 
phere, free from all traces of oxygen or other injurious elements. 
The cost of operation per ton of metal was slightly less than one-half 
of the best-known previous practice anywhere. Under prolonged 
test the furnace showed every indication of producing excellent results. 
However, like most other efficient tools, it required intelligent use, 
which the management could not see fit to give, although they 
could save the cost of the furance in a year by so doing, and, as a 
result the furnace was practically burned up and thrown out, at a 
loss of thousands of dollars for the experiment. It took ten years, 
with many installations for competition, to prove to the manage- 
ment that the human element was the keynote to the situation. 
The practice previously condemned was finally adopted, as it would 



76 STEEL AND ITS HEAT TREATMENT 

have been in the beginning were it not for the ignorance or prejudice 
of the operators and the weight given to their opinion by the man- 
agement. Hundreds of cases similar to this have been found in 
rolling mills and heat-treating and forging shops. 

The universal practice is such that there is little or no thought 
given to the improvements that can be brought about by the intel- 
ligent use of heat; and the majority of manufacturers appear to be 
satisfied to conduct their heating operations in any kind of a furnace 
in which heat can be made, with any kind of an operator to run it, 
provided he will keep it going by maintaining the supply of fuel and 
keeping an eye on the pyrometer, on the assumption that with such 
procedure he is doing all that is necessary. It invariably works out 
that the operator in effect is doing nothing more than manufacture 
pyrometer records, instead of properly heating his steel. He will 
eagerly watch the pyrometer and turn a valve so as to at least 
maintain an appearance of doing the work properly, but observa- 
tions as to the condition of the steel, in order to determine if it is 
being oxidized and if the heat is being applied alike to all pieces 
are matters that apparently receive little of his attention. 

Effect on Operation. — The operation of the furnace in the shop 
should be regarded in the same light as the stove in the kitchen. 
The furnace operators should be taught that furnaces operate on the 
same principles as an ordinary house-heating coal stove; both must 
be given the proper attention in the matter of regulating the dampers 
for the air supply and flue gases in order to accomplish the desired 
results. The importance of regulating the amount of air which is 
used for the combustion of the fuel and of regulating the flow of 
waste gases cannot be too strongly emphasized. In this respect, the 
construction and operation of the ordinary kitchen stove is, in many 
ways, superior to the majority of so-called heat-treatment furnaces. 

The average man, when he sees a kerosene lamp smoke and 
blacken the chimney, or a gas mantle puff and impair the light, will 
ordinarily recognize that something is wrong, and immediately make 
the adjustments necessary to overcome the difficulty. Both of 
these are the effects of a common cause, the improper mixture of the 
fuel and air necessary for proper combustion; and, in making such 
adjustments, whether he knows it or not, he is merely establishing 
the proper relationship between the fuel and air necessary for good 
combustion. 

Yet we will find this selfsame man, day after day, and year after 
year, operate or permit others to operate, at great expense, furnaces 



THE HUMAN ELEMENT 77 

with oil or gas which smoke and puff and pollute the atmosphere with 
hot and obnoxious gases, but never think of making the adjustments 
necessary, which are the same as those required in case of the lamp. 
Such a man is either not " on the job " or his furnace is lacking (as 
it often is) in the essentials for good combustion common to every 
household lamp or stove, whether it burn oil, gas or coal. 

The majority of men employed in heat-treating work as well 
as a large percentage of furnaces are open to this criticism, which is 
evidence of the necessity for improvement in the human element to 
the extent of either making the adjustments if provision for such is on 
the furnaces, or, at least, to insist that the furnaces be designed on 
the ABC principles of heat generation. When such adjustments 
are made with proper furnaces, the operator benefits himself by 
decreasing the heat and gases affecting his health and comfort, 
and benefits his employer in turning out better product, less scrap, 
and saving fuel and power and conserving the life of his furnace. 

The poor heating conditions which actually exist in the majority 
of shops in this country are a sad commentary on the work of effi- 
ciency, safety first and industrial betterment so prevalent at this 
time, and sustain the point that we often look but do not see oppor- 
tunities for improvement that can be made in a simple way. The 
average furnace operator appears to act on the principle that he is 
not making a good showing unless he has plenty of smoke and flame 
belching out of every crevice of the furnace — probably for the same 
reason or lack of reason responsible for the blacksmith striking two 
blows on the anvil to one on the horseshoe. It would appear only 
reasonable to assume that the existence of such conditions in the 
shop does not permit the owner of such shop to make the statement 
that one of his most important manufacturing operations, i.e., the 
heat treatment of good steel, is conducted under the best possible 
methods with the best possible furnace equipment by the highest 
grade men, or that it is even on a par with the average machine-shop 
practice. 

Necessity for Skilled Labor. — It has been stated that the average 
manufacturer will unhesitatingly invest money in anything involved in 
his processes of manufacture outside of the human element, which is 
virtually paying a premium on everything but brains. It is com- 
mon practice to install an expensive furnace, costing thousands of 
dollars, and employ a cheap, inefficient man to run it, notwith- 
standing that it is the man who controls the output and cost of 
operating that furnace. When the relationship of the human ele- 



78 STEEL AND ITS HEAT TREATMENT 

ment to the result is of less importance, as, for instance, with a 
machine press — as it surely is when compared with a furnace — then 
there will be manifested the false economy effected by the employ- 
ment of unskilled, inefficient furnace operators. In the final analysis 
the furnace is nothing more than a tool in the hands of the operator, 
and the value of the human element depends upon the amount of 
skill that must be exercised in the use of a tool, whether it is a 
hammer, a chisel, a press or a furnace. There are many cases when 
it is permissible to employ unskilled labor in connection with a 
machine, where the operation is more or less automatic and the 
operator is required to do nothing more than start or stop the move- 
ment and feed material. Such a practice, however, is foolhardy with 
a furnace, because of the paramount importance of the human ele- 
ment in the operation. It is a waste of money to install efficient 
types of furnaces, which are necessarily expensive, without providing 
for intelligent supervision over the operation in the form of at least 
one efficient man who can either operate it himself or direct its 
operation by others. The practice of employing at least one skilled 
man for such purpose is gaining headway and will undoubtedly 
continue to do so, as his labor is usually more than paid for by 
the savings effected in the cost of operation, to say nothing of 
the betterment of the product. A good furnace coupled with a 
poor operator does not make the proper combination, and when 
both are of inferior caliber, as they so often are, then it is unrea- 
sonable to suppose that the all-important heating operations are 
conducted as they should be, even though there may be some, 
but, nevertheless, weak, evidence in the form of pyrometer records 
to the contrary. 

The employment of cheap labor frequently proves to be false 
economy. One will often note two men at $3.00 a day conducting 
an operation improperly at high cost, when one good man at $5.00 
a day would conduct it properly at less cost. In one case it is a 
waste of labor and fuel, while in the other the labor is more than paid 
for by the saving in fuel alone, and everything else is clear gain. 
The real test is the cost of finished product, with due regard to quality, 
and not the price to be paid the individual for doing it. If a high- 
priced man can turn out better work at less cost than two other low- 
priced men for their individual efforts, then it would appear to be 
false economy in the long run not to give proper recognition to the 
human element; and there is a heavy penalty paid for this failure 
in. the ultimate cost of labor, fuel, power, rejected material and 



THE HUMAN ELEMENT 79 

everything else entering into the operation that is in any way affected 
by contact with the human element. These points in themselves 
should develop proper recognition of the importance of the human 
element, but paramount to these is the fact that the business of the 
customer is either held or lost by the human factor at some stage of 
the operation. 

The Value of the Furnace Operator. — If, as generally conceded, 
men are paid in proportion to their skill and the part that such 
skill plays in the make-up of a finished product, then a good annealer 
is worth more than a roller in a rolling mill, and a good man in charge 
of heat-treatment work is worth more than a machine operator in a 
shop. In one case the man operates a machine and it is a machine 
that more or less determines the result, and at any rate it is a mechan- 
ical check on the operation. In the other case, it is purely a question 
of skill, experience and judgment, with no mechanical check upon 
the major part of the operation. The furnace and all auxiliary 
appliances are but tools, and while it is necessary that they should be 
of the best, they are, nevertheless, but tools in effect. Furthermore, 
it is the judgment of the furnace operator that determines if the 
work of all that have preceded him shall be spoiled or improved 
upon, and if the time, labor, and money so spent are capitalized or 
wasted. 

Selection of Men and Equipment. — The existing standards of 
furnace design and operation are altogether too low, and there is a 
great need for improvement in the design and selection of furnaces, 
as well as in the selection of men to operate them. Most of this is 
due to lack of knowledge of the actual requirements on the part of 
the purchaser or furnace builder, or to a spirit of false economy in 
considering one type of furnace as good as another, or one fuel or 
type of furnace suitable for all heating operations, when he would not 
think of such procedure in buying steel or similar material, factory 
equipment or a motor car. 

When, far instance, a manufacturer desires to extend his power 
equipment or to develop a new process of making a stamping or a 
design of a machine for the purpose, he does not permit the operator 
of a machine to determine the details, probably on the assumption 
that the operator does not know enough about it and that the results 
can only be determined by men who have made a study of such mat- 
ters and are skilled in their execution. Yet this same manufacturer 
will, in effect, frequently permit his furnace operators to dictate the 
manner of heating a product and of designing a furnace for the pur- 



80 STEEL AND ITS HEAT TREATMENT 

pose, or, as is more common, permit him to pass final judgment on 
the efforts of others in the same direction. 

Frequently, and it may be said very often, in some of the most 
prominent plants in the country the illogical reasoning of furnace 
operators has in effect overruled the judgment of competent engi- 
neers, who, in addition to having had the advantage of sound tech- 
nical reasoning, have been backed up with proof of their ideas in 
actual practice. 

In the one instance it is a case of employing trained men to deter- 
mine methods and equipment for their execution and of operators to 
carry out the ideas of these trained men. In heating, however, the 
practice in the majority of shops would seem to indicate that the 
trained men are employed to work out the ideas of furnace operators, 
who invariably know less about the result than the machine operator. 
If it is right in one case it is wrong in the other, as the principle is the 
same. 

The man operating the furnace may mean well enough, but this 
does not alter the fact that he may not actually know what he is 
talking about; and it invariably works out that he is wrong, because 
he has not had the advantage of well-rounded experience or study 
of the principles involved in the work entrusted to his care. Prece- 
dents, habit, and long associations with any one method, coupled 
with the natural desire to resist a change, are at the root of the evil. 
Ofttimes the reasoning is based upon comparison of previous prac- 
tice in the same plant or with the practice in some other plant, but 
this is not conclusive unless it be based on a thorough knowledge of 
the principles involved and the general development in other lines 
brought about by the proper application of such principles. 

Too many men are satisfied with the conclusion — what we are 
doing is good enough, but unfortunately if such reasoning were sound 
there would be no necessity for a change in methods brought about 
by the spur of competition through the competitor who was not 
satisfied with what he was doing or with what the other fellow was 
doing, and got there first with a method superior to all, and inci- 
dentally reaped the first and best profits. 

Conclusions. — The human element in heat-treatment operations 
is important and needs attention and improvement. The best men 
available are none too good for such important work. They should 
be afforded every possible facility for comfort, handling of material 
and good furnaces which will make possible absolute control of every 
step of the operation, whether it is the making of the fire, the control 



THE HUMAN ELEMENT 81 

of the atmosphere, the flow of spent gases or the application of the 
heat to the stock. It is a combination of the man, the furnace and 
the steel that determines the result, and not any one or two of them. 
Good steel is entitled to proper treatment in good furnaces, and both 
are entitled to the services of good men to produce good results. 



CHAPTER V 

FORGING 

Forging vs. Heat Treatment.— No small percentage of the diffi- 
culty encountered in heat-treatment operations is due to improper 
forging methods, and ofttimes the heat-treatment operation is nothing- 
more than a useless effort or attempt to get something out of a forged 
piece of steel that is not actually in it. Thus, the steel man is often 
blamed for the absence of quality which he actually put in it; and 
the heat-treatment man is blamed for his lack of ability to locate 
such qualities, which he properly assumes to exist, but which, never- 
theless, the forge man- took out by poor heating, unknown to himself 
or the other two. 

It usually happens that neither the steel manufacturer, nor the 
man in charge of the forging, nor the man in charge of the heat treat- 
ment considers this point very thoroughly or investigates it properly. 
The result is that the forge man unconsciously continues to furnish 
the ground for trouble, leaving it for the other two to fight out. Much 
of the data gathered from tests and the conclusions drawn would 
appear in a different light if the very important operation of forging 
were given proper attention. 

Initial Heating. — Any heat-treatment process that does not involve 
thorough consideration and control of the initial forging operation is 
incomplete. Heat treatment begins with the heating of the steel pre- 
paratory to forging and ends with the final cooling following the last 
heat-treatment operation — assuming, of course, that the steel itself 
is in proper condition with reference to uniformity in its chemical 
analysis and homogeneity of metal when charged into the forge 
furnace. 

Many of the irregularities in heating for forging are common to 
the mill operations of heating for rolling, and a great deal of the 
difficulty due to lack of uniformity could be eliminated by a better 
control of the heating in the mill when the ingots or billets are rolled, 
and in the forge shop when the stock is formed into shape. 

The heat-treatment specialist cannot be expected to get out of a 
piece of steel or to put into it something that the forge heater has 

82 



FORGING 



83 



taken out with improper heating methods. His work is influenced 
greatly by the heating operations in the forge shop and the rolling 
mill and he properly cannot claim to have control of, or be required to 
control and assume responsibility for, the heat treatment of his steel 
unless such control extends at least to the forge shop and, if need be, 
to the rolling mill. 

This important question of initial heating has been considered 
altogether too lightly and the marked improvement that has been 
effected by special investigation along these lines indicates the 
necessity for improved methods and equipment for heating in the 
forge shop and rolling mill, neither of which have kept pace with, or 
are up to the standard of those employed for, the heat treatment of 
the steel after it is forged. 

This condition is illustrated by the test pieces shown in Fig. 53, 
together with the photomicrographs in Figs. 54 to 60, and the fol- 
lowing physical test results: 



Piece No. 


Tensile 

Strength. 

Lbs. per Sq. In. 


Elastic Limit. 
Lbs. per Sq. In. 


Elongation 
Per Cent in 2". 


Reduction 
of Area. 
Per Cent. 


Fracture. 


1 


88,000 


50,500 


14 


28.5 


angular: dead 


2 


80,500 


50,000 


11 


13.5 


burnt 


3 


96,750 


55,750 


22 


41.9 


dead: half cup 


4 


86,000 


51,250 


27.5 


39 


dead: angular 


5 


85,750 


51,750 


22 


34.1 


granular 


6 


95,900 


54,000 


12 


36.5 


granular 


7 


81,650 


54,000 


32.5 


65 


silky : full cup 



All of the shafts represented by the seven test pieces were made 
from the same heat of steel, were of approximately the same size and 
were subjected to the same amount of working during forging, and 
all were treated in the same manner at one time to meet the same 
specification. The steel represented by test piece No. 7 was properly 
forged; the fracture after heat treatment shows a perfect cup, with 
large reduction; and that the steel received the proper heat treatment 
is borne out by the photomicrograph and the physical test results. 
The first six tests indicate a wide variety of improper forging methods; 
some of the fractures were granular and showed " fire," some were 
" dead " and showed no life; all showed severe overheating. 

Such conditions are by no means exceptional even in large plants 
where there is apparently every facility in the form of equipment 
and supervision, except for heating in the forge shop. 



84 



STEEL AND ITS HEAT TREATMENT 



Heating for Forging. — Two of the weak links in forging practice 
from the metallurgical end are the lack of uniformity and the tem- 
perature of the heats. As a rule, the temperatures are altogether 
too high, with the result that, while the surface is apparently hot, 
there may be actually a " bone " on the inside. It is common 
practice to see drawn from a furnace a billet that will drip and yet, 




Fig. 53. — Influence of Forging on Subsequent Heat Treatment. 



when placed under the hammer, there will be indications of lack of 
heating in the inside. It is the inside of the bar that determines the 
physical properties of the final forging and not the outside; there is 
nothing gained in quick wash or surface heats. 

The factors so frequently mentioned — temperature, time, mass, 
surface — are just as applicable to forging work as to heat treatment. 
Slow, soft, soaking heats, affording the steel plenty of time to heat 
up, are more desirable than the quick, higher heats. The idea 



FORGING 85 

should be to maintain the temperature of the furnace as near as pos- 
sible to that actually required to soften the steel to the extent neces- 
sary for its proper shaping in relation to the capacity of the hammer or 
press, and to give it plenty of time in the furnace to soak thoroughly 
at this temperature without overheating or oxidizing the outside. 

If the judgment of the heater cannot be relied upon, as is often 
the case with men accustomed to hammering " mushy " steel or 
working on a tonnage basis, and if there is not a man continually to 
supervise the heats, then the only alternative is to install recording 
pyrometers at each furnace and that temperature maintained which 
has been found the best for the particular work in hand. If the 
proper temperature has been used, the steel thoroughly saturated, 
and the fire soft and a little high in carbon to reduce oxidation, the 
men will find that the steel actually will forge more easily and a 
greater production be obtained than under the old method. With 
proper saturation it also will be found that much lower temperatures 
may be used and that working conditions will be made more bearable. 
Such a practice not only is better for the man, but also is much better 
for the steel from the standpoint of quality. It has the added 
advantage of being less costly, because it is almost impossible to 
effect these improvements without at the same time decreasing the 
operating cost. 

Finishing Temperatures. — The finishing temperature as well as 
the maximum temperature to which the steel has been heated for 
forging has a great bearing upon the final structure of the steel and, 
therefore, upon the subsequent heat treatment. The heating and 
forging should be adjusted so that the finishing may be completed 
at temperatures as near the critical range as possible. The higher 
the initial heating and finishing temperatures, and the smaller the 
amount of working which the steel receives, the larger will be the 
grain size and the greater the difficulty encountered in the subsequent 
heat treatment. 

Figs. 61 and 62 illustrate most plainly the difference between 
proper and improper heating methods and their effect upon the 
steel. The photomicrographs were taken from large shafts of the 
same size, with the same amount of reduction, and forged by the same 
men. The same tonnage system of wages operated in each case. 
The steel of Fig. 61 represents the average result obtained by their 
old methods; that of Fig. 62 by heating in a furnace properly designed 
and operated so as to produce the soft heats, lower temperature and 
thorough saturation discussed above. 



86 



STEEL AND ITS HEAT TREATMENT 





Fig. 54.— Test Piece No. 1. 
(Bullens.) 



X100. Fig. 55— Test Piece No. 2. XlOO. 
(Bullens.) 





Fig. 56. Test Piece No. 3. X100. 
(Bullens.) 



Fig. 57. 



Test Piece No. 4. 
(Bullens.) 



X100. 



FORGING 



87 



1 £ 


■ <- .r<*. ... 




' ' V"*' 




7' 


i r ■ 


\ 




* - • i 


*■■■■ i p . 


. /.'""*"' 








'^4' V 


J 

7 


[ 




Fig. 58. Test Piece No. 5, X100. 
(Bullens,) 



Fig. 59, Test Piece No. 6. X100. 
(Bullens.) 




Fig. 60.— Test Piece No. 7. X100. (Bullens.) 



88 



STEEL AND ITS HEAT TREATMENT 




Fig. 61. — Structure of Large Shaft Improperly Heated. X100. (Bullens.) 




Fig. 62. Structure of Large Shaft Properly Heated. XJOO. (Bullens.) 



FORGING 89 

Forge Furnace Design. — The general design of forge furnaces is 
far below the standard of heat-treating furnaces and is a point usually- 
left to the forge man or to a brick-layer. It is common to see fur- 
naces hot on one side and cold on another, or hot on the top and cold 
on the bottom. Also to hear complaints on lack of ability to heat 
steel properly in a furnace in which the burners blast directly against 
the stock, which naturally keeps the stock nearest the burner cool 
and heats the pieces farther away. This is a common fault in many 
oil-fired forge furnaces, particularly those of " home-made " design, 
although it must be said that the products of many furnace builders 
are not free altogether from this criticism. 

In designing and building a forge furnace the unfortunately com- 
mon method appears to be that of building a box with working open- 
ings at one end, digging a hole somewhere in one side through which a 
burner is inserted, providing one or more holes in the roof, and calling 
it a furnace. All this is done without any regard for the generation 
and proper application of heat to the stock and without provision 
for the control of the air entering the chamber or control of the hot 
gases leaving it. 

While it is possible in many instances to introduce through one 
burner all the fuel required to produce sufficient heat, it should be 
borne in mind that in a furnace of any considerable size the heat is 
apt to localize and the blast is strongest directly in front of the 
burner. It is necessary to obtain not only sufficient heat but also 
to apply it uniformly and to eliminate the evil effect of direct blast 
close to the stock. This cannot be worked out with set rules or 
formulae; it is necessary to determine the number of burners and their 
location from a layout for each individual furnace. 

Every cubic foot of air entering the furnace should be under 
control and unless this is done it is unreasonable to expect proper 
operation, because excessive fuel consumption, oxidation, irregular 
fires and uncomfortable working conditions for the men usually follow. 
Ample space should be left on each side of the door openings so as 
to provide opportunity for good circulation of the hot gases around 
the pieces to be heated. 

Forge-furnace Operation. — The average forge heater appears 
to believe that his furnace is incomplete without one or more vents 
in the roof, and its operation imperfect unless flame is shooting out 
of the vents and working openings. On the other hand, if the fur- 
nace is designed properly and operated so as to produce soft, " lazy " 
heats the necessity for venting is demanded only by the need of bal- 



90 STEEL AND ITS HEAT TREATMENT 

ancing the pressures between the inside and outside of the furnace, 
and carrying off the gases in order that a fire can be maintained. If 
the billets project through the door opening, the opening should be 
closed with loose bricks piled around the billets. When working in 
this manner the vent will be through the openings between the 
bricks and extra vents are unnecessary. 

If the billets are introduced entirely within the chamber and the 
door closed, then special vents may be employed, but these should 
not be located in a position which will permit the gases to " short 
circuit " out of the chamber without first having passed around the 
steel in the chamber and in contact with the chamber floor. 

Whether the furnace is operated with the door up or down, one 
of the best methods is to make provision for the gases to pass out 
from the chamber at the chamber floor level and discharge from the 
furnace without blasting into the faces of the operators. Such con- 
struction decreases the effect of the cold floor or hearth by bringing 
the hot gases in contact with the chamber floor; decreases the heat 
loss through the " short circuiting " of the hot gases directly through 
roof vents occasioned in the usual construction, and finally, if the 
outlets are properly located in relation to the design and operation 
of the furnace as a whole, a more uniformly heated product will be 
produced. In practice, these vents should be closed almost entirely 
when loose brick are employed around a piece projecting through 
the furnace opening, and only partially opened when the working 
opening is covered by a door. The opening must be varied to suit 
the requirements of the fire, being greater in starting up than when 
the furnace is up to heat. 

The outlet area for the exit of spent gases should be held to a 
minimum and dampers should be provided and continually regulated 
to vary the outlet of gases. A forge furnace in this respect is entitled 
to the same consideration as a kitchen stove, not only for the proper 
generation and control of the heat, but because, in one case as in the 
other, fuel consumption varies with the outlet area. 

The Human Element in Forging. — The strongest language that 
could be employed to describe the general average heat-treatment 
equipment, the methods of heating and personnel, as they are actually 
known to exist, would be altogether too mild and ineffective for a 
proper description of the heating methods and equipment in the 
majority of forge shops in the country. As in the case of machine 
work, the design of the hammers, presses and other machine equip- 
ment has made rapid strides forward, but the two most important 



FORGING 91 

factors of the forging operation from the metallurgical end, the man 
and the furnace, have either stood still or gone backwards. 

Many well-informed and experienced men claim that a better 
quality of work was produced with the old-fashioned and com- 
paratively inefficient coal and coke furnaces, though at a higher 
cost, than at present with furnaces burning oil or gas. There appears 
to be something in this statement, particularly in view of the high 
quality of work turned out in Europe, where the use of high-speed 
machines, oil and gas fuel, and " efficiency " production methods are 
not as prevalent as here. If such a difference actually exists it can 
be traced invariably to the personnel of the men or to the design and 
operation of the furnace, because, like in most operations involving 
the skill of the operator as against the fixed movement of a machine, 
quality reflects the man and his knowledge of the work. But even 
so, we can and should be able to do better with fuels so closely linked 
with uniformity of temperature, steadiness of operation and ease 
of control. If we do not, then it is up to the man or the furnace 
and not to the hammer or the fuel, which is in itself a good argument 
for improvement of the heating and human equations in the opera- 
tions. 

It appears to be almost impossible, or, at least, commercially im- 
practicable, to build forge furnaces of proper design that will produce 
and apply heat in the manner demanded by the requirements of 
modern specifications, and expect them to stand the abuse and, in 
fact, the brutal treatment given by the ordinary forge-furnace heater. 
The design of the furnace today appears to be made to fit the man, 
although the proper procedure would appear to be that of designing 
the furnace as it should be and adapting the man to it. 

The commercial value of automobile trucks would never have 
been realized if men had not been taught how to operate and care 
for them, and the furnace and methods necessary to bolster up our 
weak forging practice will not be commercially possible until our 
. manufacturers realize the actual necessity and profit that will demand 
and follow the one factor that can retard their development — the 
human element. 

The apparent advantages of the motor truck were not side-tracked 
by the fact that it was necessary to teach men how to operate them, 
and yet there is just as much, if not more reason, why men, and if 
necessary a different type of man, should not be taught how 
properly to heat steel. The motor truck had the advantage of being 
new and free from the precedent, tradition, prejudice and custom 



92 STEEL AND ITS HEAT TREATMENT 

that hampers the development of the heating end of forge work, 
which is just as important as any other branch of heating in metal- 
lurgy and exerts much influence over other operations. 

The proper heat treatment of steel is not possible without proper 
heating in the rolling mill and forge shop, and forgings cannot be 
produced as they should be without good men and good furnaces. 
Heating is just as much of an art or trade as moulding or plumbing 
or driving a motor truck, and while trained men are demanded for 
these operations, it seems as though any kind of a man is good enough 
for the expensive operation of forging and treating. 

This factor is the weakest link in the chain of heat-treatment 
operations and must be strengthened, not only because it is neces- 
sary from the technical side, but because it is profitable from the 
commercial side. The time and money that can be saved in scrap, 
fuel, power, repairs to furnaces and other equipment, machining 
operations, lost business due to defective material, etc., should cer- 
tainly be sufficient inducement to bring about the change, the neces- 
sity for which has been too long overlooked and the possibilities too 
little realized. 

There should be continual supervision of the heating operation 
in order to check the temperature, time of heating, uniformity of heat 
throughout the chamber, the force of the fire, the amount of waste 
gases, the oxidizing effect of the fire, the consumption of fuel and 
power, and the repairs to furnaces and other equipment. Personal 
supervision is made absolutely necessary by the very nature of the 
work. Pyrometers and the like are desirable and at times necessary, 
but vital errors will creep in, even though such appliances indicate 
they do not exist. 

The practice of delegating one or more men to supervise the 
work of heating by checking the points above enumerated, under the 
direct charge of the shop foreman, is steadily gaining as it should. 
When properly followed it has invariably led to good results in the 
form of better product and saving in scrap, lost time, fuel, power, 
maintenance and better working conditions for the men. 

Forging has been neglected too long and it would' seem as though 
the technical and commercial possibilities are well worth the effort 
necessary to bring them about, 



CHAPTER VI 

THE STRUCTURE OF STEEL 

Steel.— Steel is an alloy, the principal and essential chemical 
constituents of which are iron and carbon. With these there are 
usually certain impurities, such as phosphorus, sulphur, and silicon, 
which have not been entirely eliminated during the process of 
manufacture, as well as manganese — and perhaps other alloys such 
as nickel and chrome — which may have been intentionally added 
for a definite purpose. Of the elements which go to make up ordinary 
steel, the manganese, phosphorus, sulphur and silicon — the impurities 
— generally total to about 1 per cent.; the carbon will vary from a 
few hundredths of 1 per cent, to about 2 per cent.; and the 
balance will be iron. 

Furthermore, steel is not a simple substance like copper or gold, 
but is'more like granite, in that it is made up of a number of individual 
grains (let us say) or " minerals," corresponding to the quartz, mica 
and feldspar of the granite. Thus, in steel which has cooled slowly 
from a high temperature, we have " ferrite," " cementite " and 
" pearlite." And just as the relative amounts of the quartz, mica 
and feldspar may vary in the rocks of the granite class, so will the 
relative proportions of ferrite, cementite and pearlite vary in dif- 
ferent steels according to the specific chemical composition of the 
steel as a whole. 

Cementite. — As we have stated above, the carbon and iron are 
the essential, as well as controlling, elements in the steel — and this 
is particularly true of the carbon. In steels which have cooled slowly 
from a high temperature, the carbon is first and always combined 
with adefinite amount of iron to form a " carbide of iron," corre- 
sponding to the chemical symbol FesC. This compound consists of 
6.6 per cent, carbon and 93.4 per cent, iron, and is micrographically 
known as " cementite." The balance of the iron is practically 
carbon-free and is known as " ferrite." 

Pearlite. — Now during the process of cooling at a moderate rate 
from a red heat, this cementite will form a mechanical mixture with a 

93 



94 STEEL AND ITS HEAT TREATMENT 

definite amount of ferrite, so that the resultant will contain approx- 
imately 0.9 per cent, carbon. This new constituent is called " Pearl- 
ite " and usually consists of interstratified layers or bands of ferrite 
and cementite. Pearlite is regarded as a separate and distinct 
constituent of steel, as it forms distinct " grains " when present in 
any appreciable quantity, always contains this definite percentage 
of carbon, and — as will be explained later— is always born at a definite 
range of temperatures. 

Eutectoid Steels. — From this it will be seen that a steel contain- 
ing 0.9 per cent, carbon will consist entirely of pearlite. Such steels 
are known as " eutectoid " steels, and that ratio of carbon as the 
eutectoid ratio. 

Hypo-eutectoid Steels. — Steels containing less than this eutectoid 
ratio of carbon will consist of a definite amount of pearlite, varying 
according to the carbon content of the steel proper, and the balance 
in " free " or " excess " ferrite. These steels are called " hypo- 
eutectoid " steels, as they contain less than 0.9 per cent, carbon. 

Hyper-eutectoid Steels. — Similarly, if the carbon content exceeds 
0.9 per cent, carbon, there will not be sufficient ferrite to inter- 
stratify with all of the cementite, so that these steels will consist of 
pearlite plus free cementite. Such steels are called " hyper-eutec- 
toid " steels. 

Expressing this in a different way, we may say that very low 
carbon steels are made up of ferrite with a little pearlite. With 
increase in the carbon content of the steel, the amount of pearlite 
will likewise increase, with a corresponding diminution in the amount 
of free ferrite, until at 0.9 per cent, carbon the steel will be wholly 
pearlitic. Beyond this point the amount of pearlite will decrease, 
with a corresponding increase in the amount of free cementite. 

Structure of Slowly Cooled Steels. — In slowly cooled steels we 
may therefore tell with great accuracy the approximate structural 
composition of the steel. And, vice versa, knowing the relative pro- 
portions of pearlite and ferrite or cementite, as determined micro- 
scopically, we may determine the approximate carbon content of the 
steel. 

This is represented graphically in Sauveur's diagram as shown in 
Fig. 63, in which the percentage carbon is represented by the 
abscissae and the percentage constituents by the ordinate?. 

These facts are also illustrated by the photomicrographs in Figs. 
M to 71, representing the structure of slowly cooled steel of 0.06, 
0.18, 0.32, 0.49, 0.57, 0.71, 0.83 and 1.46 per cent, carbon respectively. 



THE STRUCTURE OF STEEL 



95 



In Figs. 65 to 69 it will be seen that the pearlite (dark; structure not 
brought out by the etching) gradually increases in amount, while the 
ferrite (light) proportionally diminished. Fig. 70 shows a steel of the 
eutectoid composition in which the ferrite (dark) and the cementite 
(light) are interstratified with each other, there being practically no 
" free " ferrite such as characterized the lower carbon steels. Fig. 71 
shows the structure of a 1.46 per cent, carbon steel in which the free 
cementite (white) occurs as a network between the grains of pearlite 
(dark). In Fig. 72, showing a steel of just above the eutectoid 



100 



80 



S60 



g40 



20 







Ferrite 






-— -L^Cei 


nentite 
























Pea 


rlite 

































0.2 



0.4 



0.6 0.8 

Per Cent Carbon 



1.0 



1,2 



1.4 



Fig. 63. — Ferrite-pearlite-cementite Diagram. (Sauveur.) 



ratio, we see the first appearance of the free cementite between the 
pearlite crystals. In Fig. 73 we have this whole range represented 
by means of case carburizing a " dead soft " steel. 

Physical Properties Dependent upon Constituents. — Upon the 
relative proportions of these constituents will depend the physical 
properties of the slowly cooled steel, neglecting for the time being 
their relative arrangement. Each of these components — ferrite, 
pearlite and cementite — has certain physical characteristics with 
which we must be familiar in order to gain some idea of the proper- 
ties of such steels. 



96 



STEEL AND ITS HEAT TREATMENT 




Fig. 64— 0.06 per cent. Carbon. Approximately Pure Ferrite. X75. (Ord- 
nance Dept.) 



V ] 4f" 






*^r" 



4 i 






/ .» 



A 






--«*■• /\ i Jt-^l * ■■■■ . ? » *•» ^ 

L *? 21 & * v* * 



J» in 



Fig. 65.— 0.18 per cent. Carbon. Ferrite (White) and Pearlite (Dark). X75. 

(Ordnance Dept.) 



THE STRUCTURE OF STEEL 



97 



^vri 1 






^ii A*^ 



Fig. 66. — 0.32 per cent. Carbon. Ferrite (White) and Pearlite (Dark). X75. 

(Ordnance Dept.) 





Fig. 67. — 0.49 per cent. Carbon. Ferrite (White) and Pearlite (Dark). X75. 

(Ordnance Dept.) 



98 



STEEL AND ITS HEAT TREATMENT 




jr IG 68. — 0.57 per cent. Carbon. Ferrite and Pearlite. X75. (Ordnance Dept.) 




Fig. 69. — 0.71 per cent. Carbon. Ferrite and Pearlite. X75. (Ordnance Dept.) 



THE STRUCTURE OF STEEL 



99 




Fig. 70. — 0.83 per cent. Carbon. Pearlite. X485. (Ordnance Dept.) 




Fig. 71. — 1.46 per cent Carbon. Pearlite and Cementite (White). X75. (Ord- 
nance Dept.) 



100 



STEEL AND ITS HEAT TREATMENT 



Ferrite. — Ferrite is soft, ductile and relatively weak. It has a 
tensile strength of approximately 40,000 to 50,000 lbs. per square 
inch, with an elongation of about 40 per cent, in 2 ins. Ferrite 
in itself has no hardening power as applicable to industrial purposes. 
It is magnetic and has a high electric conductivity. Its appear- 
ance under the microscope has been shown in the photomicro- 
graphs previously mentioned — that is, as polyhedral crystals in the 
low carbon steels. 

Pearlite. — As previously mentioned, the common occurrence of 
pearlite in slowly cooled steels is in the lamellar formation, it being 




Fig. 72. — Laminated Pearlite and First Appearance (as Veins between Grains) 
of the Excess Cementite. X100. (Titanium Alloys Mfg. Co.) 



composed of alternate plates of ferrite (showing dark under the 
microscope) and cementite (showing white under the microscope). 
As will be shown later, under different rates of cooling pearlite may 
exist in other formations and dependent upon the relative arrange- 
ment of the ferrite and cementite of which it is composed; some 
of these various modifications are shown in Figs. 70 and 72. Normal 
pearlite, that is, interstratified bands of ferrite and cementite such 
as shown in Fig. 70, has a tensile strength of approximately 125,000 to 
130,000 lbs. per square inch, with an elongation of about 10 per cent, 
in 2 ins. 



THE STRUCTURE OF STEEL 



101 



Cementite. — The properties of cementite are very little known 
with the exception of its great hardness and brittleness, which are a 
maximum. Free cementite, that is, unassociated with ferrite to 
form pearlite, probably does not have a tensile strength much 
greater than 5,000 lbs. per square inch. Its ordinary occurrence 
in slowly cooled steels (carbon greater than 0.9 per cent.) is either 
as a network, such as we have seen, or as spines and needles. 




Fig. 73. — Case-carburized Steel, Showing nearly Carbonless Steel (Bottom) 
Graduating into High-carbon Steel (Top). X67 (Bullens.) 



Static Strength. — We may now sum up these facts in their relation 
to the static strength of slowly cooled steel as follows: Free ferrite 
has a minimum tensile strength with maximum ductility; pearlite 
has a maximum tensile strength with low ductility; free cementite 
confers added hardness and brittleness, with a consequent lowering 
of the tensile strength. In other words, by increasing the amount 
of pearlite in the steel, we increase the static strength but with a 



102 



STEEL AND ITS HEAT TREATMENT 



corresponding decrease in the ductility. And as an increase in the 
amount of pearlite necessarily means an increase in the amount of 
carbon, the effect of increased carbon will give the same results. 
This is shown graphically in the diagram of Fig. 74. 

Heat Treatment. — Heat treatment in general consists in chang- 
ing or regulating the structure of the steel by various methods of 



120,000 

A 100,000 

o 

c 
i— i 

& 
in 

40 p. 80,000 

,Q 

I a 

\ 30 | 60,000 

o +* 

m % 

<5 20 |» 40,000 

u 

a> 

10 20,000 




0.6 0.8 

Per Cent. Carbon. 



Fig. 74. — Approximate Influence of Carbon upon the Strength and Ductility 

of Steel. 



heating and cooling. By the term " structure " is meant (1) the 
metallographic constituents, among which are those just described; 
(2) the size of the grain; (3) the net-work. In order to understand 
the nature of these changes and their application it will be necessary 
to have a clear understanding of the mechanism by which these 
changes are brought about. 

Critical Points. — The nature of steel, as explained before, is 
complex. The structure of any particular steel may be modified 



THE STRUCTURE OF STEEL 



103 



or entirely changed by various degrees of heating, and all of which 
take place in the steel while it is in the solid condition. These 
structural changes take place at temperatures known as the " critical 
points " or " critical ranges " of the steel. These critical ranges are 
denoted by the letter " A," followed by the letter "c" (abbreviation 
for the French word " chauffage," signifying " heating ") or the 



1800 



1700 



1600 



a, 1500 



£1400 
P. 

a 



1300 



1200 



1100 

























































5 




7 






A2 












4 








2 


Al 




<i£3 




A 1-2-3 
























la 








lb 























0.20 0.40 0.60 0.80 1.00 1.20 

Per Cent. Carbon. 

Fig. 75. — Critical Range or Carbon-Iron Diagram. 



1.40 



letter " r " (" refroidissement " or " cooling "). These signs, Ac or 
Ar, are further modified by the numerals 1, 2 or 3, indicating the 
particular point referred to. Thus Acl would mean the first critical 
range passed upon heating the steel beyond a certain temperature, 
and so forth. These critical points or ranges are indicated graphic- 
ally in Fig. 75. 



104 STEEL AND ITS HEAT TREATMENT 

In considering this diagram let us devote our attention to a 
certain specific case, such as a low-carbon steel with about 0.2 per 
cent, carbon. We will also assume that the steel is in the normal 
condition resulting from slow cooling, in that it consists of about 
25 per cent, pearlite and about 75 per cent, free ferrite. We will 
also first consider what is the influence which these changes occurring 
during the critical ranges have upon the constituents of the steel. 

In the first place, practically no change in the constituents occurs 
during heating until a temperature corresponding to the lower 
critical range, Acl, is reached, which is equivalent to about 1330° F. 

In passing through this critical range there is a complete change 
in the nature and structure of the pearlite, it being converted into an 
entirely new constituent with new characteristics. This is tech- 
nically known under the generic term of a " solid solution," micro- 
graphically called " Austenite." The excess ferrite remains 
unchanged. 

Solid Solutions. — To understand better the nature of this new 
component let us consider the interaction between salt and ice. 
When these two substances are placed in contact with each other, we 
know that under suitable conditions of temperature the salt and 
ice merge into one another and so pass from the state of two separate 
substances or mechanical mixture into that of one separate substance 
or brine solution. A similar process takes place in the case of the 
pearlite, except that the resultant solution is solid instead of being 
a liquid. The individual plates of ferrite and cementite which have 
characterized the pearlite grains now merge into one another, form- 
ing this new substance or constituent, known as a solid solution. 
This new constituent, save that it is a solid and not a liquid, has all 
the properties of a liquid solution. Its original components are 
merged into a single entity, giving a complete indefiniteness of com- 
position, and with entirely new characteristics. 

Absorptive Power of Austenite.— Just as the brine solution can 
dissolve more salt or ice with increased temperature, so this solid 
solution of iron and iron carbide possesses the power of absorbing 
more free ferrite or free cementite. Therefore, as the temperature 
is raised above that of the lower critical range (Acl), and there being 
an excess of ferrite in this particular steel (0.2 per cent, carbon), 
the solid solution or austenite begins to absorb this ferrite. This 
continues progressively with increased temperature until the upper 
critical range, Ac3, is reached, or, for this particular steel, a tempera- 
ture of about 1525° F. At this temperature the last of the remaining 



THE STRUCTURE OF STEEL 



105 



excess of ferrite is absorbed by the austenite, so that above the upper 
critical range of the steel the steel is composed entirely of austenite — 
the solid solution. 

These changes are illustrated graphically in Fig. 76, It will be 
seen that the initial pearlite, comprising about 25 per cent, of the 
normal steel, changes into austenite (the solid solution) at a tempera- 
ture corresponding to that of the lower critical range, Acl, and then 
progressively absorbs the free ferrite until at a temperature corre- 
sponding to that of the upper critical range, Ac3, the whole steel con- 
sists of austenite. 




Free Ferrite 



Fig. 76. 



-Change of Pearlite and Free Ferrite into Austenite during Heating 
Carbon about 0.2 per cent. 



These same changes are shown microscopically in Figs. 77, 78, 
79, and 80. The first photomicrograph shows the normal condition 
of the steel, being made up of a small proportion of pearlite (dark), 
and a large amount of free ferrite (light) . The three other structures 
were obtained by heating this same steel to temperatures above the 
lower critical range and then " fixing " the structure obtained at 
those temperatures by " quenching." Fig. 78 shows the structure 
representative of a temperature between that of the Acl and Ac2, 
the solid solution x (dark) having increased considerably in amount 

1 Strictly speaking, the dark areas thus referred to as the " solid solution " 
are not austenite, but its transitional stage, martensite. In the ordinary carbon 
steels austenite as such cannot be retained by the ordinary methods of quenching 



106 STEEL AND ITS HEAT TREATMENT 

over that of the original pearlite as in the previous figure. Fig. 79 
represents the structure obtained at a temperature somewhat under 
that of the upper critical range, Ac3; in this case it will be noted 
that the solid solution covers nearly the whole field, there being but a 
small amount of the free ferrite (white). The structure representa- 
tive of heating to slightly above the upper critical range is shown in 
Fig. 80; it will be seen that the free ferrite has now been entirely 
absorbed by the solid solution. Also note the extremely refined 




Fig. 77.— Normal 0.2 per cent. Steel Carbon. X100. (Bullens.) 

structure, as we shall have occasion to refer to this particular feature 
a little later. 

Allotropic Modification of Ferrite. — Associated with these crit- 
ical ranges there is also a change in the allotropic 1 form of the 
ferrite (iron). Thus pure ferrite (as distinguished from the ferrite 

(as will be explained under the chapter on Hardening) , but changes into martens- 
ite. Martensite, however, is also a solid solution, and for the purposes of explana- 
tion in this chapter — in order not to complicate matters — we will consider it 
permissible to use the term as indicated. 

1 Sauveur defintes allotropy as " suggesting marked and sudden changes in 
some of the properties of a substance occurring at certain critical temperatures, 
without any change of state or of chemical composition." 



THE STRUCTURE OF STEEL 



107 




Fig. 78.— Low-carbon Steel Quenched between Acl and Ac2. X 100. (Bullens ) 




Fig. 79. — Low-carbon Steel Quenched a Little below Ac3. X100. (Bullens.) 



108 



STEEL AND ITS HEAT TREATMENT 



associated with cementite to form pearlite) in its normal condition 
is called " alpha "-ferrite or " alpha "-iron, and is characterized 
by extreme ductility and magnetic properties. Upon heating this 
alpha-ferrite to a little over 1400° F., corresponding to the critical 
range Ac2, the iron becomes practically non-magnetic and is then 
known as " beta "-ferrite or " beta "-iron. Upon further heating to 
a temperature above the upper critical range, Ac3, there is still 
another change in the allotropic modification of the iron, it being 
known as " gamma "-ferrite; this gamma-iron is slightly softer than 
the beta modification. Gamma-iron has the property of being able 







Fig. 80. — Low-carbon Steel Quenched above A3. X100. (Bullens.) 



to dissolve carbon or iron carbide, a characteristic which is not held 
by alpha-iron. 

Merging of the Critical Points. — Now, by referring to the carbon- 
iron diagram in Fig. 75 it will be noted that at the eutectoid ratio of 
carbon, that is, at about 0.9 per cent, carbon, 1 the three critical ranges 
Al, A2, and A3, merge into one. That is, steels consisting of pearlite 
alone, when heated to a temperature beyond this point, will change 
directly into the solid solution austenite, which will consist of a 
solution of carbide (or carbon, according to some authorities) in 

1 The eutectoid ratio on the chart is given as 0.85 per cent, carbon. Accord- 
ing to the authority selected this ratio will vary between 0.8 and 0.9 per cent, 
carbon; but the more recent tendency is to adopt 0.90 per cent. 



THE STRUCTURE OF STEEL 109 

gamma-iron. Similarly, as normal pearlite always represents this 
eutectoid ratio, the same change of pearlite into a solid solution of 
carbide in gamma-iron will always occur at this temperature in ordi- 
nary carbon steels irrespective of the carbon content of the steel as 
a whole. 

Changes in Heating Different Steels. — With this explanation 
clearly in mind, we may now refer back to the example of the 0.2 
per cent, carbon steel and more fully explain the changes which take 
place in. the constituents. Under normal conditions, this steel will 
consist of pearlite plus alpha-ferrite. Upon heating through the 
Acl range, the pearlite will change into austenite, the iron of which 
will be in the gamma modification; the free ferrite will still remain 
in the alpha condition. Upon further heating through the zone 
marked " 2 " on the diagram Fig. 75, the austenite will begin to 
absorb the free ferrite. Upon passing through the Ac2 range the 
balance of the free ferrite will pass from the alpha modification into 
that of beta-f errite ; the steel as a whole will be hard and non-mag- 
netic. Upon further heating (zone 3) the remnant of the beta free 
ferrite will be gradually absorbed, so that on passing through the 
critical range, Ac3, the whole steel will be in the condition of austen- 
ite (zone 5), or a solution of iron carbide (or carbon) in gamma-iron. 

In a similar manner we might explain the changes in constituents 
which take place upon heating normal steel with any carbon up 
to that of the eutectoid ratio. With a carbon content somewhere 
between 0.3 and 0.4 per cent, (varying according to different author- 
ities) it will be noted that the A2 and A3 ranges merge into one, 
known as A2-3. 

In a manner analogous to the absorption of free ferrite by the 
solid solution in the hypo-eutectoid steels, the free cementite will be 
absorbed in the case of the hyper-eutectoid steels, the final solution 
taking place at a temperature range indicated by the line Acm. 
The only difference, and that a practical one, is that the solution of 
the free cementite takes place more sluggishly than the solution of 
the free ferrite of the lower carbon steels. 

The Ar Ranges. — Corresponding critical changes take place 
upon cooling slowly from above the upper critical range, except that 
they occur in the reverse order and with opposite effect. On account 
of the molecular inertia, however, we find that these critical ranges 
(of cooling, Ar3, Ar2, Arl, etc.) are a number of degrees below the 
temperatures at which they appeared on heating. This difference 
is dependent upon length of exposure and the temperature to which 



110 STEEL AND ITS HEAT TREATMENT 

the steel was subjected, the rate of cooling, and, more particularly, 
upon the influence of the alloying elements which may have been 
added to the steel. Some of the alloys, if present in sufficient amount, 
will cause the recalescent points to fall below normal temperatures, 
and are the basis of air-hardening steels and similar compositions. 

Changes on Slow Cooling. — Upon slow cooling from above the 
upper critical range, the solid solution will commence to reject the 
excess ferrite (or, of course, the excess cementite in the case of hyper- 
eutectoid steels) as the temperature decreases from Ar3 to Arl. 
The reverse changes in the physical nature and properties of the 
iron occur at the critical ranges during cooling as those previously 
noted under heating. When the lower critical range is reached, the 
excess ferrite or cementite will have been entirely rejected, and as 
the steel passes downward through this range (or point), the solid 
solution — now containing 0.9 per cent, carbon — will change into 
pearlite. Under similar conditions of cooling, the original steel and 
the present heated and cooled steel will have the same structure. 

Refinement. — Before leaving the subject of the influence which 
heating through these various critical ranges has upon the structure 
of the steel, there are a few points which we wish to mention briefly 
concerning refinement. Again assuming that the steel is in the 
normal condition, no change will take place in the structure until the 
temperature has been raised at least to that of the lower critical 
range. At this temperature the original pearlite grains are com- 
pletely changed and will possess that maximum refinement which 
the formation of the austenite can impart — that is, complete refine- 
ment. If the steel has a carbon content other than that of the eutec- 
toid ratio (i.e., contains free ferrite or free cementite), the steel as a 
whole will not be refined; the excess ferrite or cementite will remain 
unaltered and the steel will retain its original grain-size. This is 
brought out by a comparison of Figs. 77 and 78. Complete refine- 
ment of the steel as a whole will not result until the steel has been 
heated to a temperature slightly over that of the upper critical range, 
as a comparison of Figs. 79 and 80 will prove, and as is evident from 
previous discussion. A clear understanding of these principles must 
be had, as they form the basis of many of the heat treatment proc- 
esses which will be later developed. 

Grain-Size Beyond Ac3.- — As the temperature is progressively 
raised above the critical range, a gradual coarsening of the aus- 
tenite grains occurs. This increased size is not only a function of the 
temperature, but also of the length of time at which the high tern- 



THE STRUCTURE OF STEEL 



111 



perature is maintained. The practical application of the principles 
noted in this and the previous sections will be considered in the chap- 
ter on Annealing. 

Network. — The third factor in the structural changes taking 
place upon heating is the effect of temperature upon the network. 
All hypo-eutectoid steels in the normal condition are made up of 
pear lite with a varying amount of excess ferrite, the latter decreasing 
with the increase in carbon content. From our study of the inter- 




Fig. 81. — Microstructure of Cast-steel Ingot as Cast. X75. (Ordnance Dept.) 
Tensile Strength, 77,000. Elastic Limit, 39,000. Elongation, 10.5. 
Red. of Area, 16.9. 



nal mechanism by which the constituents of the steel are formed 
by slow cooling, we know that the pearlite forms the basis of the 
structure, the ferrite being rejected by the solid solution (pre-pearlite). 
Being thrown out to the boundaries of these austenitic grains, the 
excess ferrite forms a network around these grains. Upon reheat- 
ing, this network is gradually absorbed, its final absorption taking 
place upon passing the upper critical range. This change is similar 
to that explained previously under the description of the action of 
the excess ferrite. 



112 



STEEL AND ITS HEAT TREATMENT 




Fig. 82.— Microstructure of Cast Steel Ingot Forged to 1450° F. X75. (Ord- 
nance Dept.) Tensile Strength, 83,500. Elastic Limit, 50,500. 
Elongation, 27.5. Red. of Area, 43.3. 




Fig. 83. — Microstructure of Steel Subjected to Cold Work, and Showing Dis- 
tortion of Grain. X50. (Ordnance Dept.) 



THE STRUCTURE OF STEEL 



113 




Fig. 84. — Hammer-hardened Steel, 0.46 per cent. Carbon. X300. (Savoia.) 




Fig. 85.— Effect of Cold Rolling on 0.20 per cent. Carbon Steel. X60. 

(Bullens.) 



114 



STEEL AND ITS HEAT TREATMENT 




Fig. 86. — Effect of Punching upon Structure of ^-in. Chrome-nickel Steel 
Plate. Hole Downwards and at Right. X50. (Bullens.) 




Fig. 87. — Machining Strains on Surface of Mild Steel. (Brearley.) 



THE STRUCTURE OF STEEL 115 

The Effect of Work on Grain-Size. 1 — Steel cooled slowly and 
undisturbed from a high temperature will show a coarsely granular 
or crystalline structure, and the size of the grain is a function of the 
temperature and time during which the material is held at the maxi- 
mum temperature, and the rate at which the material is cooled. In 
large masses of material the structure will be coarser in the center 
than at the surface, due to the difference in rate of cooling. In 
order to overcome this difference and at the same time produce a 
homogeneous, uniform material, the steel is worked during the period 
at which grain growth would ordinarily take place. Steel which has 
been hot-worked down to the Arl point will show a finer grain, and 
will be stronger than the same steel slowly cooled without work, 
and will at the same time show high ductility. Examples of steel 
worked and unworked are shown in the photomicrographs of Figs. 
81 and 82. 

Steel which has been worked below the Arl range — that is, cold- 
worked — will show considerable distortion of grain, as is illustrated 
by Fig. 83, and may even become hardened, Fig. 84. Cold rolling 
frequently develops a weak, laminated structure, as is shown in Fig. 
85. Even punching or machining operations may greatly affect 
the structure, examples of which are given in Figs. 86 and 87. 

1 In part from Bulletin 1961, Ordnance Dept. 



CHAPTER VII 

ANNEALING 

Annealing. — Annealing, in its commercial application, may have 
for its purpose any or all of the following aims: (1) to " soften " the 
steel and thus put it in condition for machining or to meet certain 
physical specifications; (2) to relieve any internal stresses or strains 
caused by previous hardening or elaborating operations; (3) to obtain 
the maximum refinement of the grain in combination with large 
ductility. 

Thus, depending upon the results desired, commercial annealing 
will consist of a heating operation carried to some predetermined 
temperature — although not necessarily over the critical range — to 
produce the results desired in items 1 and 2 previously noted, and 
followed by a moderately slow cooling of the metal from that tem- 
perature. True or full annealing requires a heating to above the 
upper critical range of the steel, followed by suitable cooling. 

Heat Application. — In the abstract, annealing would appear to be 
but a suitable correlation of the elements of temperature, time, mass, 
and surface. But in actual practice the success to be obtained 
in annealing (or in any heat-treatment process, for that matter) 
depends not only on an understanding of the technique of the proc- 
ess, but also upon the judgment and skill of the furnace operator 
in applying the basic principles which may be derived from a 
consideration of the above factors. Thus it is the man who deter- 
mines the manner of placing the charge in the furnace, of regulating 
the flow and composition of. the hot gases, of determining when the 
steel has been heated uniformly and thoroughly. Particular stress 
should be laid upon the necessity for getting heat to the center and 
bottom of the charge, not only for the sake of uniform annealing, but 
also to shorten the time of saturation and lessen the time of exposure 
of the top and outside edges of the charge to the heat and influences 
of the chamber atmosphere. 

It has been shown that a uniform chamber temperature does not 
mean necessarily a uniformly heated product; that a circulation of 
heat through the mass is more desirable than the mere application 
from the outside; that, with the same chamber uniformity, it is pos- 

116 



ANNEALING 117 

sible to vary the quality of the anneal by the manner in which the 
steel is placed in the heating zone. It is advisable to raise the charge 
above the furnace floor or hearth upon suitable blocks or supports, to 
separate each piece from the other, and to avoid localized heating 
through overloading. It is only by such means that there will be 
provided an opportunity for the circulation of the hot gases through 
the charge. 

Elemental Considerations. — Fundamentally an annealing opera- 
tion consists of two well-defined and co-ordinated phases, the heating 
phase and the cooling phase. Just as a photographer first must 
expose his plate to the chemical action of light and then by other 
chemical means develop and fix the image on the plate, so must the 
annealer first expose his steel to the action of heat in order definitely 
to impress upon it those characteristics which will be capable of 
development and of being brought to summation and completion in 
the second or cooling phase. 

Regardless of whether the phase being considered is one of heating 
or of cooling, of heat absorption or heat emission, each is concerned 
with a suitable correlation of the basic factors of temperature and time. 

The temperature to which the steel is to be heated is that at which 
the desired change in grain size, micro-constituents, or both will take 
place. The temperature of the cooling phase is defined by the 
transformation range of the steel to be annealed. 

Aside from the general question of initial heating, the time ele- 
ment in the heating phase is one of duration at the predetermined 
temperature, of the saturation necessary, and is dependent metal- 
lurgically upon the microstructure of the steel as received for anneal- 
ing, the mass and surface. The time element in the cooling phase is 
an expression of the rate at which the steel shall be cooled through 
the critical range and is determined by the microstructure desired 
in the steel, the mass and surface. 

Initial Heating. — Slow, careful and uniform heating is always 
advisable regardless of the chemical composition or physical condi- 
tion of the steel. Heating to such temperatures as are common in 
general annealing practice necessarily results in more or less change 
of physical condition or molecular readjustment, and the greater 
the hardness, brittleness and amount of internal strain in the metal, 
the greater will be the deleterious effect of such heating. Thus 
objects of intricate design, or with varying cross-sections, or steel 
in a hard, brittle condition, should be given the greatest care in heat- 
ing in order that the release of any strains shall not cause warpage 



118 STEEL AND ITS HEAT TREATMENT 

or otherwise injure the metal. Such pieces should never be placed 
directly in a hot furnace, but should be given a careful pre-heating. 

ANNEALING HYPO-EUTECTOID STEELS 

Microscopic Changes. — In the previous chapter we have explained 
that, in the ordinary cast, rolled or forged sections (pearlitic in 
character), there is virtually no change in grain size or in constit- 
uents during the heating to a temperature below that of the lower 
critical range Acl . That is, there is no refinement of the steel. 

As the temperature passes the Acl range there occurs the complete 
change of the pearlite to the solid solution, giving the maximum 
refinement to the austenite. 

Passing through zone 2 (Refer to Fig. 75) the excess ferrite is 
progressively absorbed by the solid solution. This absorption is the 
slower the greater the carbon until the carbon nears 0.85 per cent., 
but is offset by the fact that the amount of free ferrite decreases as 
the eutectoid ratio is approached. 

Upon passing through the critical range Ac2 we have the forma- 
tion of beta iron with no apparent change between the relative 
grain-size of the alpha ferrite and beta ferrite grains. The same 
absorption of the excess ferrite continues progressively, but with 
increased sluggishness (due to the supposed properties of beta ferrite) . 
This applies to steels with say 0.12 to 0.30 per cent, carbon. In 
the very low carbon steels Howe 1 sums up the probable changes 
during this period in a provisional proposition that (a) if initially 
fine-grained the steel coarsens, though only very slowly; (6) if 
initially coarse-grained it refines slowly; (c) to coarsen again upon 
long exposure to these temperatures. 

The changes taking place through zone 2 continue through 
zone 3, although more slowly. If the rate of heating through this 
range of temperatures is comparatively slow, there will be a complete 
absorption of the remaining ferrite just before Ac3 is reached. 
Under ordinary circumstances final absorption will occur on passing 
through the Ac3 range. 

The Upper Critical Range.— As the steel passes the upper critical 
range there is the complete refining of the grain, it becoming very 
fine and almost amorphous. As the temperature increases beyond 
this range the grain-size coarsens, causing a diminution in the 

1 H. M. Howe, " Life History of Network and Ferrite Grains in Carbon Steel," 
Proc. A. S. T. M., Vol. XI, 1911. 



ANNEALING 



119 



strength of the steel. The effect upon the physical properties of 
the steel is great. The tensile strength is increased somewhat as 
the temperature advances. The elastic limit rises until a point is 
reached about 175° to 200° F. over the upper critical range, after 
which it then decreases. The elongation and reduction of area 
decrease very rapidly. These changes in the physical properties 
are shown graphically in Fig. 88 in which the results obtained by 
heating a 0.40 per cent, carbon, basic open-hearth steel to a definite 
temperature and then slow-cooling with the furnace are plotted 



, 68,000 



£66,000 



62,000 



36,000 



j 34,000 



I 32,000 



30,000 







































rfli 


























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48 g 

!3 

bo 

44 g 

3 

40 2S 
36 2* 
22 
20 
18 
16 



1,200 1,300 1,400 1,500 1,600 1,700 1,800 

Degrees Fahrenheit 

Fig. 88. — Effect of Annealing Temperature on Physical Properties. 



against the temperatures. It will be noted that the softest and 
most ductile steel is obtained at approximately 1475° F., which is 
about 50° over the upper critical range. 

Heating Over the Upper Critical Range. — The effect of heating 
beyond the critical range is well developed by the series of photo- 
graphs (by Howe) shown in Figs. 89, 90, 91, 92 and 93. The steel 
(0.40 per cent, carbon, 0.16 per cent, manganese) was heated to the 
temperatures indicated, held at those temperatures for ten minutes, 
and then cooled in air. There is a difference in grain-size between 



120 



STEEL AND ITS HEAT TREATMENT 



Effect of Heating beyond Ac3. 
0.40 per cent. Carbon Steel Heated at Temperatures Indicated for Ten Minutes and 

Am Cooled. 




Fig. 89—1472° F. X40. (Howe.) Fig. 90.-1652° F. X40. (Howe.) 




Fig, 91.-1832° F. X40. (Howe.) Fig. 92—2012° F. X40, (Howe.) 




Fig. 93.— 2192° F. X40. (Howe.) 



ANNEALING 



121 



Effect op Heating beyond Ac3. 
0.40percent.!Carbon Steel Heated at Temperatures Indicated for Ten Minutes and 

Furnace Cooled. 




1 Stt^sfia? 









Fig. 94.-1472° F. X40. (Howe.) Fig. 95.-1652° F. X40. (Howe.) 





ii SSf^F s ¥ r -~ J BT' 









Fig. 96.-1832° F. X40. (Howe.) Fig. 97.-2012° F. X40. (Howe.) 




Fig. 98.-2192° F. X40. (Howe.) 



122 



STEEL AND ITS HEAT TREATMENT 



that cooled from 1472° F. and from 1652° F., showing that anneal- 
ing should never be carried very far beyond the upper critical range 
or Ac3 point unless for special reasons. As the high temperatures 
are successively raised to 1832° and 2012° the grain-size becomes 
noticeably larger, until at 2192° the steel is overheated. These 
photomicrographs also exhibit the effect of air cooling upon the 
structure, in that it develops, in small pieces, a distinct net-work or 
cellular structure. The effect of heating beyond the upper critical 
range is also brought out in an analogous manner by Figs. 94, 95, 96, 
97 and 98, except that in this case the steel has been cooled slowly 
(furnace cooled) from the specific temperatures. 

Temperature of Heating. — Reduced to lowest terms, the true or 
full annealing operation requires the production of an entirely new 
crystalline structure, the constituents of which shall be of the smallest 
grain-size attainable; this operation should also eliminate all internal 
strains and stresses. As previously described, this new structure 
is given birth at a temperature known as the " upper critical range " 
of the steel. The exact temperature 1 will depend upon the chemical 
composition of the steel, and, more particularly, upon the carbon 
content. As this transformation does not occur suddenly, but 
usually covers a range of some 25° to 50°, it is customary to adopt 
a temperature of about 50° over the upper critical range as the 
proper annealing heat. For straight-carbon steels these may be 
given roughly as shown in the chart in Fig. 99. The upper critical 
range is located approximately by the dash line on the chart. 

The temperatures recommended by the American Society for 
Testing Materials 2 are as follows: 



Range of Carbon Content. 



Range of Annealing 
Temperature. 



Less than 0.12 per cent. 
0.12 to 0.29 
0.30 to 0.49 
0.50 to 1.00 



1607° to 1697° F. 
1544° to 1598° F. 
1499° to 1544° F. 
1454° to 1499° F. 



1 Methods for determining the critical ranges are described in Chapter 
XXI. 

2 It will be noticed that the temperatures recommended by A. S. T. M. are 
distinctly higher — especially for the tool-steel grades — than those advised by 
the author. In the light of my own experience, and that of others, I believe 
that the lower the temperature which can be used to give the desired results, 
the greater will be the maximum efficiency of the annealed steel. 



ANNEALING 



123 



Rate of Heating. — Studying the heating phase from the practical 
aspect there is another factor to be considered — that of bringing 
the whole mass of the steel to the proper temperature uniformly. It 
is self-evident that the center of a large mass of steel, such as loco- 
motive axles, shafts or steel blooms, will lag in temperature behind 
the exterior. In other words, it is the tendency of the core to be con- 
siderably lower in temperature than the shell or outside of the steel. 

1700 



1600 



11500 



£•1400 

a 



1300 



1200 





















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0.2 



0.3 0.4 0.5 0.6 
Carbon Content, Per Cent. 



0.7 



0.8 



P 



Fig. 99. — Annealing Range for Carbon Steels. 



It is then a common procedure to raise the temperature of the fur- 
nace beyond the proper annealing heat in order to drive the heat 
to the center of the steel to be annealed. This is a great mistake. 
It is far better to take the extra time required to heat more slowly 
as the proper temperature is neared, thus bringing the steel to a 
uniform temperature throughout. If this were not done, the exterior 
of the piece might be carried beyond the proper temperature — and, in 
general, a needlessly high temperature is injurious and tends to 
recoarsen the grain. 

Expressing this matter in a different way, we may say that the 



124 STEEL AND ITS HEAT TREATMENT 

furnace in which the metal is being heated for annealing should in 
no case be run at a higher indicated temperature than the maximum 
temperature to which the metal itself is to be heated. To illustrate : 
A piece of steel, such as a die-block, heats, cools and decarbonizes on 
the corners first. The life of the entire piece of steel is no greater 
than the life of the corners. If the die-block is placed in a furnace 
which is at the final working temperature, the corners are apt to 
reach the final temperature long before the major part of the mass. 
If the temperature is higher, as it is frequently in a futile attempt to 
gain time, the corners are overheated before the center of the mass 
is saturated to the proper degree. From this commonplace example 
there should be indicated the necessity for slow, soaking heats in 
order to prevent overheating the corners of the metal, and further, 
the necessity of soft, hazy heats, to prevent . oxidation or decarburi- 
zation of the exposed edges. 

Time of Heating. — The time element in annealing requires that 
the whole mass of the steel (1) shall be heated uniformly throughout 
at the proper temperature, and (2) shall be maintained at that tem- 
perature for a period sufficient to give birth to the new grain structure 
and to relieve all internal stresses. Assuming a proper method of 
heat application, the duration of heating is ordinarily determined 
by the mass — the maximum size of section — and by the surface 
exposed to the heat. We will show later, however, that this time fac- 
tor is also qualified most strongly by the initial structure of the steel. 

For the determination of the various rates of heating of specimens 
of different sizes to various furnace temperatures, experiments by 
M. E. Leeds : were made with round specimens of normal open- 
hearth carbon steel approximating 0.5 per cent, carbon, and ranging 
in size from 2 ins. to 12 ins. in diameter, by 24 ins. long. Each 
specimen was heated to four temperatures, namely, 1000°, 1200°, 
1400° and 1600° F. During the time of heating a continuous record 
was kept of furnace temperatures, etc. " As would be expected, the 
smaller specimens heat more rapidly than the larger. The relation 
between the size of specimen and the time of heating to various tem- 
peratures is brought out in the curves, Fig. 100. Except in a very 
general way, this information could not be used as a guide to heating 
practice, as the rates would vary with the size of furnace and probably 
with other conditions. The time of heating for a specimen of any 
size is less when it is brought up to 1600° F. than when brought up to 

1 M. E. Leeds, A. S. T. M., June Meeting, 1915. 



ANNEALING 



125 



1200° F., and less for 1200° F. than for 1000° F., although it is 
greater for 1400° F. than for any other temperature. It is more 
difficult to account for the fact that the higher temperatures are 
attained more rapidly than the lower ones. This fact, however, 
appears to be clearly demonstrated. It may be that the specimens 



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Fig. 100.— Curves Derived from Rate of Heating. (Courtesy of Leeds & Nor- 

thrup Co.) 



received a large amount of their heat by radiation from the furnace 
walls. The heat transfer by radiation between two bodies at dif- 
ferent temperatures is proportional to the difference between the 
fourth powers of their absolute temperatures, and so for a 100° dif- 
ference in temperature between furnace wall and test specimens, at 



126 



STEEL AND ITS HEAT TREATMENT 



1600° F., the heat transfer would be at a higher rate than for the 
same temperature difference at lower temperatures." 

For general purposes it may be assumed that, after the furnace 
has attained the indicated temperature desired, about one-half 
hour to one hour should be allowed for heating and saturation for 
every one inch of section. This is a very general statement and must 
be modified according to the experience of the individual heat treat- 
ment man and methods of heat application. 




Fig, 101. — Fine Network Structure. 0.4 per cent Carbon Steel, X100. 

(Bullens.) 



Behavior of the Excess Ferrite on Cooling. — As previously ex- 
plained, the cooling of a hypo-eutectoid steel from a temperature 
above that of the upper critical range to a temperature below the 
lower critical range will cause the solid solution or austenite to attempt 
to reject the excess ferrite to the grain boundaries. The ordinary 
full-annealing operation generally will accomplish this precipitation 
of the excess ferrite, and if the cooling through this temperature 
range has been retarded sufficiently, the excess ferrite will coalesce 
into definite grains. 

Thus the cooling phase, depending upon the correlation of the 



ANNEALING 



127 



elements of time, mass, and surface, will produce a structure vary- 
ing from that of a fine network (Fig. 101) — indicating that very 
little of the excess ferrite has been precipitated because the rate of 
cooling was comparatively rapid; through the intermediary stages 
of coarse network (Fig. 102) and partial coalescence (Fig. 103); 
to that showing complete coalescence of the excess ferrite into indi- 
vidual grains (Fig. 104) — indicative of extremely slow cooling. The 
pieces of steel from which the photomicrographs were taken were of 




Fig. 102. — Coarse Network Structure. 0.4 per cent Carbon Steel. 

(Bullens.) 



X100. 



different sizes, from the same ingot, and annealed at a temperature 
high enough to bring out more forcibly by means of a comparatively 
large grain-size the definite structures desired; a finer grain could 
have been obtained by the use of a lower temperature. 

With the same temperature of heating, if the elements of time- 
mass-surface in that heating phase have been adjusted correctly, it 
is evident that the time-mass-surface elements of the cooling phase 
will control the final structure of the annealed steel. 

Cooling in the Furnace. — Regardless of the size (mass-surface 
elements) of that piece, if the time element of cooling can be pro- 



128 



STEEL AND ITS HEAT TREATMENT 



longed sufficiently, complete coalescence of the excess ferrite always 
may be obtained. From the practical standpoint it may be said 
that the production of a structure showing complete coalescence of 
the excess ferrite always requires a cooling of the steel in the fur- 
nace — or its equivalent; and further, that the temperature-drop 
of that furnace (i.e., the temperature-time elements) during cooling 
must be in inverse proportion to the size of the piece (i.e., the mass- 




Fig. 103. — Partial Coalescence. 0.4 per cent Carbon Steel. X100. (Bullens.) 



surface elements). This may be illustrated by the three following, 
widely divergent examples. 

1. It would be good practice to assume that the commercial 
annealing of shafts 10 in. in diameter would require a large furnace 
of heavy construction, having a small radiation loss, and, therefore, 
a very slow temperature-drop after the fires had been shut down and 
all doors, vents and dampers closed. With such a method of cooling 
it then would be immaterial whether the pieces to be annealed were 
10 in. or 1 in. in diameter: in either case the steel would be cooled 
very slowly and complete coalescence of the excess ferrite would 
result. In this illustration the method of cooling indicates the 
results, regardless of the size of the piece. 



ANNEALING 



129 



2. For the heating of a few pieces of 1-in. bars it might be con- 
sidered good practice, under certain plant conditions, to use a small 
furnace of light construction, which would have the tendency to 
cool down rapidly when the fires were shut down. Cooling small 
pieces in this furnace (it having a radiation loss not in inverse pro- 
portion to the size of the steel) would not be retarded sufficiently to 
produce a state of complete coalescence of the excess ferrite, but only 
of partial coalescence, as is shown by Figs. 94-98. In this case 




Fig. 104. — Complete Coalescence. 0.4 per cent Carbon Steel. X 100. (Bullens. 



it is the mass-surface elements of the steel, as well as the method of 
cooling, which indicate the final structure to be expected. 

3. When the size, design and construction of the annealing fur- 
nace are in proper adjustment with the mass and surface of the steel 
exposed to the heat, complete coalescence will result from slow-cooling 
the steel in and with the furnace, as is indicated by the structure 
resulting from the furnace cooling of a 4-in. bar, shown in Fig. 106. 

On account of the great variation in heat radiation during the 
cooling of steel with different mass-surface factors in furnaces of 
different sizes, design and construction, the exact rate of cooling 
required to produce complete coalescence is one to be determined 



130 STEEL AND ITS HEAT TREATMENT 

Effect of Annealing on Physical Properties. 0.4 per cent Carbon Steel. 




Fig. 105.— Forged from 12 in. to 4 in. Square. X 100. (Bullens.) T. S., 18,600, 
E.L., 32,500; EL, 28%; R.A., 45%. 




Fig. 106.— Annealed, Furnace Cooled. X100. (Bullens.) T.S., 67,700; E.L. 
35,400; EL, 35%; R.A., 55.5%. 




Fig. 107.— Annealed, Air Cooled. X100. (Bullens.) T.S., 76,250; E.L. 
38,150; EL, 34%; R.A., 56%. 



ANNEALING 



131 





Fig. 108.— 10-in. Shaft. 0.4 per cent 
Carbon. Annealed, Air-cooled. 
X100. (Bullens.) 



Fig. 109.— 10-in. Shaft. 0.4 per cent 
Carbon. Annealed, Air-cooled. 
X100. (Bullens.) 




Fig. 110.— 10-in. Shaft. 0.4 per cent 
Carbon. Annealed, Air-cooled. 
X100. (Bullens.) 




Fig. 111.— 10-in. Shaft. 0.4 per cent 
Carbon. Annealed, Air-cooled. 
X100. (Bullens.) 



132 STEEL AND ITS HEAT TREATMENT 

largely by experiment under the distinctive working conditions. It 
may be said that the slower the cooling of the steel from the annealing 
temperature through the transformation range, the greater will be 
the degree of coalescence of the excess ferrite. 

Other Slow-cooling Methods. — If the objects are of large size, 
an approximation of furnace cooling may be obtained by removing 
the hot steel from the furnace and covering with some blanketing 
substance and slow conductor of heat such as lime, ashes, or sand. In 
some instances, cooling small pieces in lime may often recult in a 
slower cooling than may be obtained in the furnace. When a large 
tonnage of steel must be annealed and there is not sufficient furnace 
capacity to permit of cooling in the furnace, a pit lined with brick 
or metal plates, and fitted with coverplates may be used. By this 
method the hot steel is delivered immediately from the furnace to the 
pit and covered with ashes; cooling by this method of pit-annealing 
often is slower than cooling in the furnace itself if the latter is not 
properly designed and constructed. 

Structures Produced by Air Cooling. — The structure which will 
be produced in steel with definite mass-surface factors, air-cooled 
after proper saturation at a given annealing temperature, may be 
determined with much exactness, because the relation between the 
heat radiation of the steel and the temperature of the air are definitely 
fixed and well known. Thus the fine network structure of Fig. 101 
(a 1-in. bar) or of Figs. 89 to 93 is characteristic of the air-cooling of 
small pieces; the part coalescence of Fig. 103 and Figs. 108-111 of 
large sections; and that of Figs. 102 and 107 of intermediate sections. 
The fine network structure can be produced in large sections only 
with cooling by artificial means — such as quenching. The furnace 
cooling of small pieces, when not sufficiently retarded, will often 
produce results equivalent to the air cooling of larger pieces, as is 
indicated by a comparison of Figs. 94-98 with Figs. 107 or 111. 

Effect of Cooling Rate on Physical Properties. — Ordinarily a full 
annealing should produce the maximum refinement possible, such as 
is illustrated by Figs. 124 and 126. A structure like Fig. 124 is 
almost amorphous and is indicative of a nice adjustment of the 
annealing temperature, coupled with thorough saturation at that 
temperature. With such structures it is almost impossible, under 
low magnification, to distinguish the difference microscopically 
between air-cooling and cooling in the furnace. The difference, 
however, generally is shown by the results of physical tests. 

In order to illustrate the effect upon the physical results of vary- 



ANNEALING 



133 



ing the rate of cooling, a 12-in. square billet of 0.4 per cent, carbon 
steel was forged into a 4-in. square bar. One piece of the bar was 
cooled slowly in the furnace from the annealing temperature, while 
another piece was cooled directly in the air. The annealing temper- 
ature selected was such as would produce a small grain-size, and yet 
not so fine that the difference between the structures could not be 
recognized under the microscope at a magnification of 100 diameters. 
The physical test results follow : 



Condition of Steel. 


Structure. 


Tensile 
Strength. 
Lbs. per 

Sq. In. 


Elastic Limit. 

Lbs. per 

Sq. In. 


Elongation 

Per Cent, in 

2 Inches. 


Reduction of 
Area. 


Forged 

Annealed, furnace- 
cooled 

Annealed, air-cooled. 


Fig. 105 

Fig. 106 
Fig. 107 


78,600 

67,000 
76,250 


32,500 

35,400 
38,150 


28 

35 
34 


45 

55.5 
56 



These results indicate that the slower the cooling the lower will be 
the tensile strength and the greater the ductility. A similar effect is 
also obtained by double annealing, as is shown by the test results on 
page 143. 

Air-cooling gives what might be termed a " reasonable " strength 
and large ductility, as is shown also by the physical test results from a 
shaft of 0.4 per cent, carbon, 10 ins. in diameter (photomicrographs 
Figs. 108 to 111), air-cooled: 



Location of Test. 


Tensile 

Strength. 

Lbs. per Sq. In. 


Elastic Limit. 
Lbs. per Sq. In. 


Elongation 

Per Cent, in 

2 Inches. 


Reduction of 
Area, Per Cent. 


Outside 

Center 


78,000 
78,000 


46,750 
42,500 


32.5 
32 


54.6 
54 6 







These last four photomicrographs show also the progressive change 
of structure from the outside of the shaft (Fig. 108) to the center 
(Fig. Ill), indicating another effect of mass-surface upon the cooling 
phase of annealing. 

Effect of Initial Structure. — The previous discussion has been 
concerned mainly with the annealing and production of a definite 
structure in steel which has been subjected to proper heating and 



134 STEEL AND ITS HEAT TREATMENT 

working in previous elaborating processes such as rolling, forging, 
etc. Regardless of mass, if the steel to be full-annealed has a " rea- 
sonable " grain size before annealing, such as is illustrated by the 







Fig. 112.— Frame Steel as Rolled. ~ X60. (Bullens.) 

photomicrograph of Fig. 105, the time element will be normal and an 
ordinary heating at the proper temperature will produce a small 
grain size. In other words, there should be no great difficulty in 




Fig. 113.— Frame Steel Partly Annealed. X60: (Bullens.) 

producing the desired results by annealing with an ordinary satura- 
tion at the proper temperature — a simple anneal. 

If such were the extent of the demands made upon the knowledge 
of the heat-treatment man, the work would, with some experience, be 



ANNEALING . 135 

comparatively easy. Unfortunately, however, existing plant con- 
ditions often make heat-treatment work a more complex problem, 
for reasons such as were given in the chapter on " Forging," or which 
may be caused by some other elaborating operation even properly 
performed. We will find, in cases in which the steel has been sub- 
jected to severe mechanical working — as in Fig. 112, or in large 
castings and forgings showing ingotism — as in Fig. 132 and 129, 
respectively, or in large sections which previously have been over- 
heated severely — as in Figs. 122 and 125, that the time of saturation 
may pass beyond normal limits and become a factor of magnified 
importance, Special annealing methods, or annealing requiring 




Fig. 114.— Frame Steel Fully Annealed. X60. (Bullens.) 

several days of " soaking/' may be required to bring about the neces- 
sary molecular changes in the steel. The reasons for this may be 
explained by the slowness with which absorption proceeds or by 
the phenomena of diffusion and equalization. 

Diffusion. — It seems that the greater the internal stress upon the 
steel the greater is the amount of intermolecular lag or final release 
of this stress behind the actual change of constituents. That is, 
even though a totally new structure may be set up by the anneal- 
ing heat, there remains for a considerable length of time a ten- 
dency of the new structure to return, upon slow cooling, to the 
stressed condition of the original, even though the constituents them- 
selves may be those born at the new temperature. 

For an explanation of this let us hark back to our former simile 
of the salt and brine solution. When a grain of salt is dissolved by 



136 



STEEL AND ITS HEAT TREATMENT 



the brine, it is the solution in the immediate neighborhood of the 
salt crystal which acts as the solvent and not the entire volume of 
the brine solution. In time, however, the dissolved salt will eventu- 
ally diffuse through the whole body of brine and the brine will then 
be of equal composition throughout. Now a similar process is going 
on in the steel when the solid solution (austenite) is absorbing the 
excess ferrite, and it will be found that complete absorption may not 
mean complete diffusion or equalization. The process of equalization 




Tig. 115. — Coarse Grain Not Eliminated by Simple Annealing. X 100. (Bullens.) 



goes on with the rise in temperature. If the passage through tem- 
peratures under that of the upper critical range is only slow enough, 
a large part of the diffusion will have occurred by the time Ac3 is 
reached. In order that there may be complete diffusion, and there- 
fore, complete grain-refining, the sojourn at a temperature approxi- 
mating Ac3 must be long enough for this complete diffusion of the 
absorbed excess ferrite and, therefore, of the solid solution. 

This point is illustrated metallurgically by Figs. 112, 113 and 114. 
These are photomicrographs taken from tests made upon -^--in. thick 
chromium nickel steel plates for automobile frames: Fig. 112 shows tho 
structure of the steel as rolled; Fig. 113 shows the steel after an ordi- 



ANNEALING 



137 



nary annealing at a temperature somewhat above the upper critical 
range; and Fig. 114 shows the same steel after a long anneal at the 
same temperature. It will be noticed that the steel in Fig. 113 has 
taken on approximately the same structural constituents as in the 
fully annealed piece, as shown in Fig. 114, but that it still remains in 
the stressed condition of Fig. 112, even though the annealing temper- 
atures were the same in both cases. The reoccurrence or reformation 
of these laminations or other stressed structures is due to the fact 




Fig. 116. — Passing through the Al Range. X100. (Bullens.) 



that the complete effacement by equalization had not taken place. 
In other words, it means that where these stressed areas occur the 
carbon content, as a whole, is less than in the rest of the mass. Where 
ferrite predominates, as in the lower carbon steels, there will the mass 
more easily coalesce into what may be termed " milky- ways " 
(Howe). It is important, therefore, if a soft steel, free from all 
internal strains and stresses is desired, that a sufficient length of time 
be allowed for the permanent elimination of these intermolecular 
strains, before and during cooling. 

The necessity for supersaturation or special heating methods for 
securing diffusion and equalization not only is required for the 



138 STEEL AND ITS HEAT TREATMENT 

removal of stressed structures caused by severe mechanical working, 
but also for the elimination of the effect of severe overheating. 
This condition is illustrated by the case of a large forging which had 
been made under conditions of improper heating — due to ignorance, 
and of insufficient working resulting from lack of proper forging 
equipment. After the forging had been put through a full-annealing 
process with normal saturation, it was found by microscopic examina- 
tion that little or no refinement had taken place. The annealing 




v&MJm&Ll&r^ 



Fig. 117.— Structure above Al. X100. (Bullens.) 

was repeated, but with similar results; the steel still showed the 
coarse grain of Fig. 115. A J -in. square test strip then was cut from 
the forging, one end heated white-hot, the strip immediately quenched 
in water and then examined under the microscope. 

The group of unusual photomicrographs thus obtained is shown m 
Figs. 115 to 120. The characteristic, progressive absorption, between 
the lower and upper critical ranges, of the excess ferrite is apparent. 
The commencement of the transformation range is shown across the 
middle of Fig. 116, indicated by Al and the arrow. Fig. 117 repre- 
sents a slightly advanced stage in the transformation, showing the 
hard matrix of the solid solution (martensite, in this instance) , and 



ANNEALING 



139 




Fig. 118. — Structure at A3 Range (middle); Unabsorbed Ferrite (bottom) and 
Non-diffused Ferrite (top). X100. (Bullens.) 




Fig. 119. — Structure above A3, Showing Lack of Diffusion. X100. (Bullens.) 



140 



STEEL AND ITS HEAT TREATMENT 



the softer free-ferrite. Fig. 118 shows a section evidently corre- 
sponding to the structure at a temperature approximating that of the 




Fig. 120. — Martensite Showing Ghost-outlines of Non-Equalization. X100. 

(Bullens.) 

Ac3 range, since only a small amount of the original excess ferrite 
(white) remains unabsorbed. This photomicrograph is particularly 




Fig. 121. — Test Pieces Showing the Stages in the Annealing of Pverheated Steel. 



interesting in that there is noted the distinct ghost-outlines x of the 
absorbed, but not diffused, ferrite. Examination at high magnifica- 

1 The specimen was polished and etched repeatedly in order to be absolutely 
positive that the ghost-outlines were not caused through carelessness in the 
metallographic work. 



ANNEALING 



141 



tions showed clearly the fact that an area of non-diffusion surrounded 
the unabsorbed ferrite. Figs. 119 and 120 are photomicrographs of 
sections higher in the heating range (over Ac3) and the same char- 
acteristic lack of diffusion and equalization is quite apparent, even 
in the absence of ferrite: High magnifications showed that this 
darker structure had all the metallographic characteristics of martens- 
ite, and failed to resolve further the faint network. It was only in 
sections corresponding to a markedly higher temperature that the 




Fig. 122. Overheated Steel with Large Grain Size. X100. (Bullens.) 



photomicrographs indicated complete diffusion and equalization. 
The importance of these phenomena clearly is indicated, and the 
solution of problems involving their practical application will now 
be considered. 

Refining Overheated Steel. — Overheating in previous operations 
usually results in a type of structure similar to those shown in Figs. 
122, 125 or 129. 

The structure of Fig. 122 indicates overheating by the extremely 
large grain-size. Overheating likewise is shown by the granular 
character of the fractured test-piece (the left in Fig. 121), and by the 
physical test results of: Tensile strength, 104,200 lbs. per sq. in.'; 



142 



STEEL AND ITS HEAT TREATMENT 




Fig. 123.— Structure after a First Anneal at 1450° F. X100. (Bullens.) 




Fig. 124.— Structure after a Second Anneal at 1450° F. X100. (Bullens.) 



ANNEALING 



143 



elastic limit, 58,220 lbs. per sq. in. ; elongation, 7 per cent, in 2 ins. ; 
reduction of area, 3 per cent. 

Small pieces having an initial structure of this nature usually can 
be refined by a simple anneal, but with large pieces a double anneal 
may be required. 

The three photomicrographs, Figs. 122, 123 and 124, and the test 
pieces of Fig. 121, represent the stages in the double annealing of a 
shaft 8 ins. in diameter, made of 0.4 per cent, carbon steel. It will 
be noticed (Fig. 123) that the first heating at 1450° F., followed by 
cooling jn the furnace, served to break up the coarse grain (but 
without full refinement) and to redistribute the ferrite and pearlite; 
it also raised the elongation from 7 to 22.5 per cent., and the reduction 
of area from 3 to 37 per cent. A second anneal, at 1400° F., followed 
by cooling in the furnace, produced an almost amorphous structure, 
as is shown in Fig. 124; the tensile test gave a full-cup fracture (see 
Fig. 121), and a very high ductility. The physical properties of the 
three tests were as follows: 



Structure. 


Photo- 
micrograph. 


Tensile 

Strength. 

Lbs. per 

Square Inch. 


Elastic Limit. 

Lbs. per 
Square Inch. 


Elongation 

Per Cent, in 

2 Inches. 


Reduction 

of Area. 

Per Cent. 


As forged 


Fig. 122 

• Fig. 123 

Fig. 124 


104,200 
90,500 
60,000 


58,220 
50,300 
29,100 


7 
22.5 
43 


3 


First anneal 

Second anneal 


37 
69 



It is probable that a single anneal with a very long saturation 
would have refined this steel without the necessity of a double 
anneal, but a double heating and cooling through the critical ranges 
undoubtedly is commercially quicker and produces greater refine- 
ment and higher ductility. 

Refining Severely Overheated Steel. — A second characteristic 
initial structure before annealing is shown in Fig. 125. It is evident 
that this steel not only has been severely overheated, but that the 
spine-growth of the excess ferrite also indicates a temperature of at 
least 2300° F. 

In the case of small pieces an unusually thorough saturation at 
the ordinary annealing temperature will break up entirely the old 
structure and refine the steel. This is shown by Fig. 126, repre- 
senting the result of annealing the 1-in. bar of Fig. 125, in the manner 
indicated above. 



144 



STEEL AND ITS HEAT TREATMENT 




Fig. 125.— Severely Overheated SteeL X100. (Bullens.) 




Fig. 126.— Severely Overheated Steel, 1-in. bar, Refined by One Long Anneal. 

X100. (Bullens.) 



ANNEALING 



145 




Fig. 127. — Severely Overheated Steel, 10-in. Shaft, after Ordinary Double Anneal 

X100. (Bullens.) 




Fig. 128— Steel of Fig. 125 after Second Double Anneal. X100. (Bullens.) 



146 



STEEL AND ITS HEAT TREATMENT 



The annealing of large sections with a similar initial structure 
presents much more difficulty than did the refinement of a structure 
like that of Fig. 122. The result of an ordinary double annealing of a 
10-in. shaft which had an initial structure similar to that of Fig. 125, 
is shown in Fig. 127 : The grain size still is large, although the physical 
results indicated good ductility. A second double anneal produced 
the results indicated in Fig. 128: Although the steel largely is refined, 




Fig. 129. Maximum Overheated Steel. X100. (Bullens.) 



there still is the tendency of large grains or " islands " here and there 
to remain unaffected. 

Refinement of Maximum Overheated Steel. — A third phase of 
initial structure is represented by Fig. 129, and indicates that the 
steel is burnt, or, at least, has been heated to the maximum tempera- 
ture possible without being burnt. A structure like this in small 
pieces of steel is very difficult to refine, while in sections of large 
dimensions it often is economically impracticable. The photo- 
micrograph of Fig. 130 shows the result obtained from a 10-in. shaft 
after eight annealings at various temperatures, totaling some 200 
hours of heat saturation. And even after such repeated heating and 



ANNEALING 



147 




Fig. 130.— Steel of Fig. 129 after Eight Anneals. X100. (Bullens.) 




Fig. 131.— Steel of Fig. 107 Annealed for Strength— Ductility. X 100. (Bullens.) 



148 



STEEL AND ITS HEAT TREATMENT 



cooling the original, distinctive arrangement of the ferrite has not 
been eliminated entirely. 

Special Double Anneal. — When the initial structure previous to 
annealing is of such character that a simple anneal, or a double anneal 
at normal temperature, will not refine the steel, the use of a special 
double anneal often is found of practical benefit. In such a case the 
first heating is carried to a temperature considerably (200° or even 
300° F.) in excess of the upper critical range; after thorough satura- 




Fig. 132.— Characteristic Ingot Structure. Casting. X100. (Bullens.) 



tion, the steel is cooled to under the lower critical range, followed by a 
regular full-anneal. The technique of the first anneal is based on 
the fact that the molecular changes take place the more rapidly the 
higher the temperature. By this means the time element may be 
shortened for the reason that diffusion and equalisation will take 
place more rapidly. The commercial success in breaking up initial 
overheated structures is dependent upon the degree to which the 
molecules can be prevented from returning to that state, that is, of 
obtaining the maximum diffusion and equalization in as short a time 
as possible. Such high-temperature annealing will produce a larger 
grain size than would a more normal temperature, but if the over- 



ANNEALING 



149 



heated structure thus can be broken up, the mere refining of a large 
grain size structure is comparatively easy, as has been discussed 
previously. 




Fig. 133. — Sorbitic Pearlite (left) and Laminated Perlite (right.) X1000. 

(Price.) 

Whether or not the steel shall be air cooled or furnace cooled after 
the first heating is a question which must be determined largely by 




Fig. 134. — Sorbitic Pearlite Passing into Normal Pearlite. X 1000. (Van Tassel 

and Price.) 

experience. If the saturation at the high temperature has been 
sufficient to obliterate entirely the initial structure and to overcome 
the tendency to return to a state of strain, then it is immaterial what 



150 



STEEL AND ITS HEAT TREATMENT 



method of cooling is used. But, if complete diffusion and equaliza- 
tion has not taken place, it would be considered good practice to air 
cool the steel in order that as little opportunity as possible shall be 




Fig. 135. — Finely Laminated Pearlite. X1000. (Van Tassel and Price.) 

given for the old structural outlines to reappear. On the other hand, 
it might be argued that, with an initial structure similar to that of 
Fig. 129, effort should be made to coalesce the excess ferrite into 




Fig. 136. — Laminated Pearlite Passing into Massive Pearlite. X1000. (Van 

Tassel and Price.) 



individual grains (Fig. 104 and 106) and by such means attempt to 
eliminate the long, banded streaks of ferrite. The method and 
equipment for handling the stock in and out of the furnace, the value 
of the time required by furnace cooling and the distinctive plant 



ANNEALING 



151 



conditions will have a strong bearing upon the method of cooling to 
be used. 

Annealing for Strength-ductility. — Provided that the initial 
structure of the steel is such that an ordinary full-anneal will produce 
a small grain-size, the best combination of ductility, strength and 
machining properties which can be obtained without an actual quench- 
ing operation will be obtained by the following annealing operations: 

Heat to slightly over the Ac3 range, air cool to below the Arl 
range, and reheat at a temperature slightly below the Arl range 
(about 1250° F.), holding the steel at this temperature until it is 
heated uniformly throughout, and then slow cool. In fact, the last 
cooling may be made in the air if desired, as there will be little or no 
change during cooling from a temperature under the lower critical 
range. 

By permitting the steel to air cool after the first heating, to a tem- 
perature below the lowest critical range, advantage is taken of any 
" hardening effect " or retardation in the transformation of austenite 
into coalesced ferrite and pearlite. This effect will increase with the 
percentage of carbon and the smaller the mass-surface factors of the 
piece, so that there will be the tendency to form a mass of irresolvable 
and intermixed pearlite and ferrite known as " sorbite." The re- 
heating to a temperature below the lower critical range, if not pro- 
longed unduly, will neither change the grain size nor allow of the 
coalescing of the excess ferrite or of the individual constituents 
of the pearlite. At the same time, as will be explained under the 
chapter in " Toughening," there will result an excellent combination 
of large ductility, good strength and machining properties. 

The effect of taking the air-cooled, annealed bar of Fig. 107 and 
reheating, as above indicated, is shown in Fig. 131, and by the fol- 
lowing physical test results: 



Annealed: air cooled. 
Reheated to 1250° F. 



Tensile 

Strength. 

Lbs. per Sq. In. 



76,250 
74,400 



Elastic Limit. 
Lbs. per Sq. In- 



38,150 
33,000 



Elongation 

Per Cent, in 

2 Inches. 



34 
37.5 



Reduction 
of Area. 
Per Cent. 



56 
67.5 



Annealing for Maximum Ductility. — If, regardless of tensile 
strength, the maximum ductility is desired, and the ordinary full- 
anneal will produce a small grain size, the method should be as follows: 



152 



STEEL AND ITS HEAT TREATMENT 



Anneal at a temperature slightly over that of the Ac3 range, slow 
cool to under the Arl temperature, reheat to slightly over the Acl 
temperature, and cool in the furnace. 

Effect of Rate of Cooling upon the Pearlite. — Not only does the 
rate of cooling from the annealing temperature have a very great 
effect upon the network, grain structure and excess ferrite, but also 
upon the characteristics of the pearlitic constituent of the steel. The 
rate of cooling through the lower critical range, at which the trans- 
formation of the solid solution into pearlite is effected, will so change 
the arrangement of the ferrite and cementite constituents of the 
pearlite that widely varying physical results may be obtained in this 
manner. 













Fig. 137.— Massive Pearlite. X1000. (Van Tassel and Price.) 



As we will explain later, the austenite does not directly change 
into pearlite, but passes through a series of transition constituents 
with varying physical properties. The majority of these, however, 
are not retained in the steel through methods of cooling other than 
quenching (which may or not be followed by a reheating), so that 
we need consider only the very last transition, sorbite. This com- 
ponent sorbite represents the last stage of the transition austenite to 
pearlite, and in which the individual particles of ferrite and cementite 
are just on the verge of coalescing. Sorbite, or sorbitic pearlite, is 
noted for its combination of high tensile strength (i.e., in comparison 
with the later phases of pearlite) and ductility, and when obtained by 
annealing is found generally in small pieces which have been air 
cooled from the annealing temperature (see Figs. 90-93). The 
appearance of sorbitij pearlite just before, and after, passing into 



ANNEALING 153 

finely laminated pearlite is shown at the left and right, respectively, 
of Fig. 133; this photomicrograph — at 1000 diameters — was taken 
from a 2-in. round bar of 0.93 per cent, carbon, annealed at 1400° F. 
and very slowly cooled. 

The effect of the rate of cooling upon the structure of the pearlite 
is brought out clearly by the photomicrographs of Figs. 134, 135, 136, 
and 137, taken from 0.73 per cent, carbon, tool steel bars differently 
annealed. The structure of Fig. 134 shows the changing of a sorbitic 
pearlite into finely laminated or normal pearlite, with the cementite 
element semi-segregated ; Fig. 135, finely laminated pearlite; Fig. 136, 
laminated pearlite with completely segregated cementite, and Fig. 
137, laminated pearlite passing into massive pearlite, with the 
cementite and ferrite each coagulating. 

Tool Steel Annealing. — The annealing of hypo-eutectoid tool 
steel may be broadly grouped under two headings, dependent upon 
the initial condition of the steel and upon the results desired. Tool 
steel which has been carefully hammered is undoubtedly strength- 
ened by tins mechanical elaboration; a full annealing — that is, heat- 
ing at a temperature over the critical range — will entirely destroy 
the results of the forging operation. If it is therefore desired simply 
to anneal the steel in order to put it in suitable condition for machine 
work — that is, to soften it and at the same time to retain the bene- 
ficial effects of the forging — the annealing operation should be 
carried out at a temperature less than that of the critical range, or in 
the neighborhood of 1200° to 1250° F. On the other hand, if it 
is desired to obtain the finest grain size possible, the maximum 
softness, and to entirely eliminate any previous heating or forging 
work, the annealing should be carried out at a temperature slightly 
over that of the critical range, or in the neighborhood of 1400° F., 
dependent upon the composition of the steel in question. 

Protection of Steel. — One of the vital points in obtaining a satis- 
factory steel after annealing is the protection of its surface. Steel 
when heated beyond a low-red heat exhibits a great tendency to 
oxidize or scale, this action increasing, in the presence of oxygen, 
with the temperature and the length of time involved. This con- 
dition will exist in furnaces operated so as to produce sharp heats, 
instead of soft, slightly hazy, reducing atmospheres. Decarburiza- 
tion to a depth of \ to \ inch is not a rare occurrence where improper 
combustion and heat application is the rule. If, due to poor furnace 
design and worse operation, such conditions do exist, in order to 
produce a clean surface it will be necessary to protect the exposed 



154 STEEL AND ITS HEAT TREATMENT 

surface of the steel in some manner. Tool steel is usually annealed 
by placing in a tube, packing carefully with charcoal, and then closing 
the ends of the tube with caps or luting with clay. 

On the other hand, the prevention of excessive oxidation or the 
scaling of the metal during the heating process is a simple thing with 
the proper furnace design and operation. Assuming such a design, if 
the furnace is operated so as to produce soft, hazy heats such as we 
have previously mentioned, there should be no occasion for packing 
the steel in charcoal or other such substances, unless the surface 
appearance is important and will warrant the extra cost for tubes 
and the extra time, fuel and labor incident to their use. This state- 
ment is made not as one of theory, but as one of actual practice. 
The results which have been obtained in practice along these lines 
have been very gratifying and illustrate the economy which follows 
proper furnace design and operation. It has been demonstrated 
that a large proportion, if not the major part, of such oxidation as is 
likely to occur with proper furnaces and average care in operation 
is created during the cooling process after the fires have been shut off. 
This is due, no doubt, to the air which reaches the steel after the out- 
ward pressure in the chamber has been released. In practice this 
condition has been offset largely by placing old railroad ties or pieces 
of wood in the chamber, with the idea of creating a non-oxidizing 
atmosphere around the steel while it is cooling. 

Box-Annealing. — For the protection of larger masses or a number 
of smaller pieces, " box-annealing " is often resorted to. This par- 
ticularly applies to cases where a finished surface must not be 
injured. The steel is placed in a rectangular pot or box made of 
cast iron or of plates riveted together. This box may or not be 
lined with some refractory substance such as silica brick. The metal 
is then carefully packed with some material such as ground mica, 
sand, charcoal, charred bone or leather, lime, etc. If the steel is 
low in carbon a carbonaceous or carbon monoxide generating sub- 
stance must not be used, for a slight case-hardening action would 
take place. In the case of higher carbon steels, and especially of 
tool steels, reducing agents may be used, although it is better to mix 
the charcoal with clean ashes. Sand and ground mica are probably 
the most satisfactory of the simple non-reducing, refractory materials. 
The cover is then placed on the box and the box with its contents is 
charged into the furnace and given the proper degree and duration of 
heating. The box should be raised from the floor of the furnace so 
that the hot gases may have opportunity for circulation around it. 



ANNEALING 155 

When properly heated throughout, the box may be removed from the 
furnace and allowed to cool to atmospheric temperature. 

Stead's Brittleness. — We have previously stated that practically 
no change occurs below the Acl range if no previous hardening of the 
steel has taken place. The one exception is that of very low-carbon 
steels and is due to the fact that steels very low in carbon behave 
more like pure or carbonless iron, there being but small percentages 
of cementite (and therefore pearlite) to influence the grain-size. 
Upon heating such steels through the upper part of zone la (refer 
to diagram in Fig. 75), a distinct coarsening of the ferrite grains 
occurs, this being a function of time as well as of temperature. 
Steels of such carbon held at say 1100° F. for a considerable length of 
time will develop such coarsening of grain-size as to make the steel 
unfit for commercial use if any degree of strength is required. This 
phenomenon is known as " Stead's Brittleness." With steels of 
greater carbon content the increased pearlite so operates upon the 
molecular structure of the steel that practically no change occurs 
until the Acl range is reached. 

Summary of Annealing Methods. — The following charts give a con- 
densed summary of the methods of annealing previously discussed ; they 
should be used only in conjunction with a careful consideration of all the 
elements involved and should be altered to suit distinctive conditions. 

REFERENCE KEY TO ANNEALING METHODS 
Key. Temperature 

A Ac3+50° F. 

B Ac3+200°to300°F. 

C Acl +25° F. 

D Acl-50°F. 

Saturation 
N Normal. 

P Prolonged. 

Cooling 
Q Air. 

R Retarded. 

Initial Structure 
Character. Illustration Reference. Page Reference. 

I. Reasonable Fig. 105 133 

II. Overheated Fig. 122 141 

III. Severely overheated Fig. 125 143 

IV. Maximum overheated. . . .Fig. 129 146; 148 
V. Cold worked Fig. 112 

VI. Castings Fig. 132 

X Repeat treatment indicated if results of previous treatment not satis- 
factory. 



156 



STEEL AND ITS HEAT TREATMENT 



ANNEALING METHODS 



Steel. 



Results Desired. 



Small Sections. 



Large Sections. 



I. 

II. 

III. 

IV. 

V. 
VI. 

I. 

I. 
I. 

VI. 



Commercial Full-annealing (maximum 
refinement and large ductility) 

Commercial Full-annealing (maxi- ( 
mum refinement and large ductility) I 

Commercial Full-annealing (maxi- / 
mum refinement and large ductility) I 

Commercial Full-annealing (maxi- 
mum refinement and large ductility) 

Commercial Full-annealing (maxi 
mum refinement and large ductility) . 

Commercial Full-annealing (maxi 
mum refinement and large ductility) . 

Good machining only 

Maximum ductility and refinement . . . 

Strength-ductility 

Commercial annealing 



ANR 

APR 

APRov 
ANQ -ANR 

BNQ-ANR 

APR 

ANR 

DNQ 
ANR- CNR 
AN Q- DNQ 
ANR 



ANQ 

ANQ -ANQ 

or ANR -ANR 

BNQ-ANQ 

XANQ 

BPQ-APQ 

XAPQ or 

XBNQ-ANQ 

APQ 

APR 

DNQ 
ANR -CNR 
ANQ -DNQ 

APR 



ANNEALING HYPER-EUTECTOID STEELS 

Critical Ranges. — Strictly speaking, hyper-eutectoid steels have 
two critical ranges: the A 1.2.3, at which — on heating — the pearlite 
changes into the solid solution; and the A cm range, at which — on 
heating — there is the final solution of the excess cementite — just as 
in hypo-eutectoid steels the Ac3 range represents the solution of the 
last of the excess ferrite. However, on account of the relatively 
small proportion of free cementite in the ordinary hyper-eutectoid 
steels, and also because there is a large increase in grain-size upon 
heating to the Ac. cm range — the temperature position of the latter 
increasing very rapidly with increase in the carbon content — the 
Ac. cm range requires but little practical consideration and the 
majority of the annealing operations are more intimately connected 
with the principal critical range Ac 1.2.3. 

Commercial Annealing. — Similarly to hypo-eutectoid steels, the 
annealing of high-carbon steels may have for its object any or all of 
the following factors: (1) the release of internal strains and stresses 
set up by previous operations, (2) the softening of the steel to place 
it in a suitable condition for machining, (3) the entire change of 
structure. 



ANNEALING 157 

The first item may be accomplished by a simple reheating at 
temperatures below those of the critical range. The second and 
third items are more complex in their solution, as the form in which 
the excess cementite may exist is one of the governing factors. 

If the mass of the steel is in the sorbitic state, as may generally 
be expected in the usual tool steel, satisfactory results (the softening 
of the steel for machining, and relieving the internal strains) may be 
obtained by an annealing at a temperature slightly under that of the 
principal critical range, or at about 1250° to 1300° F. This heating 
should not be prolonged for such length of time as may cause the 
excess cementite to coagulate, but only until the steel has been thor- 
oughly and uniformly heated throughout. 

On the other hand, if it is desired to obtain the complete change 
of structure, and to refine the grain (previously coarse), it will be 
necessary to heat to a temperature at least in excess of the Ac 1.2.3 
range (about 1340° F.). For steels with a carbon content approx- 
imating 0.9 per cent., such heating will accomplish the complete 
change of structure and give the finest grain-size obtainable through 
annealing. For steels with a carbon content considerably in excess 
of the eutectoid ratio the annealing may be done at similar tempera- 
tures, provided, however, that the excess cementite is more or less 
in solution in the sorbite. 

Incidentally, if the condition stated under (3) is desired, and it 
will warrant the expense, the best method is first to oil quench from 
a temperature somewhat over the Ac 1.2.3 range, and subsequently 
anneal at a temperature just below that range. 

Normalizing. — If the steel to be annealed has the free cementite 
existing as network or spines, which would make the steel difficult of 
machining, annealing at the usual temperatures (Ac 1.2.3) will not 
affect this cementite: it will simply refine the ground-mass. In 
order to eliminate this free cementite, it will be necessary first to 
normalize or quench the steel from a temperature above that of the 
Ac.cm range. That is, air cooling from a temperature of say 1750° 
or 1800° F. will not permit of the reformation of coagulated cement- 
ite. The second annealing may then be carried out at a temperature 
of 1375° with the refining of the grain size and complete softening of 
the steel as a whole; this second heating should be just as short 
as possible in order to prevent the reformation of the free 
cementite. 

Spheroidizing the Cementite. — The above method may be further 
modified by reheating to a temperature slightly under the lower 



158 STEEL AND ITS HEAT TREATMENT 

critical range instead of over it. The objection to this is that the 
steel will not be refined, but will possess the large grain size charac- 
teristic of the high temperature. On the other hand, the lower 
annealing temperature will entirely prevent the formation of the free 
cementite as either spines or as a network. Instead, it will be found 
that the excess cementite will be thrown out, under these conditions, 
as little nodules or " spheroids " if the reheating temperature is just 
about at the end of the lower critical range; or, under certain con- 
ditions, the whole mass of the steel may be called " granular," if such 
a term is permissible. Further reference to this spheroidal forma- 
tion of cementite, as obtained by a double " quenching," is given 
under Chapter XI. Spheroidal cementite in annealed steels may 
also be obtained by very slow cooling through the end of the Arl 
transformation: cementite in this condition is a great help in the 
machining of high-carbon steels. 



CHAPTEB VIII 
HARDENING 

Hardening.' — Fundamentally, the operation of hardening in- 
volves two operations of change in temperature : heating and cooling. 
The function of the heating is (1) to obtain the best refinement, and 
(2) to obtain the formation of the " hard " constituents of the steel. 
Having done this, the steel must then be held in this condition by 
very rapid cooling — that is, by quenching in some medium such as 
water or oil. Associated with both the heating and rapid cooling 
there must be as great a degree of uniformity as is possible. 

Changes on Heating. — Steel, when properly hardened, should 
show no trace of the original structure, such as coarse grain size, 
network, unabsorbed ferrite (in hypo-eutectoid steels), or any other 
peculiarities of untreated steel. If such are present in the hardened 
steel it goes to prove that the operation was not properly carried out. 
Further, if the structure of the steel has not been suitably changed or 
developed by the heating operation, it most assuredly will not be 
altered for the better by subsequent quenching. The most that such 
quenching can do is to retain the characteristics which the heating 
has developed. 

An attempt has been made graphically to illustrate these facts 
in the chart in Fig. 138. Column 1 (at the left) represents a normal, 
0.4 per cent, carbon, pearlitic steel (at the bottom of the column), 
and the structural changes taking place in that steel as it is pro- 
gressively heated through and beyond the critical ranges. For the 
present it is assumed that the structure and micrographic constit- 
uents obtained by heating to various temperatures, such as A to E, 
may be retained by quenching, as illustrated by columns II to VI. 

Thus heating to a temperature A, under that of the lower critical 
range, will produce no change in the original steel, which consists of 
pearlite (the cross-hatched circles) and ferrite (the black area). 
The quenching likewise will produce no change, as is illustrated by 
Column II. 

Heating to a temperature B, slightly over the lower critical range, 
will change the pearlite to the solid solution (represented by 

159 



160 



STEEL AND ITS HEAT TREATMENT 



the dotted area), but without affecting the free ferrite. Quenching, 
column III, will therefore produce a semi-hardened steel— since the 
solid solution is the " hard " constituent— with a refinement of the 
" ground-mass " (the original pearlite) only. 

Heating to a temperature C, between the lower and upper critical 
ranges, will effect a progressive absorption by the solid solution of 




Fig. 138. — Changes in a 0.4 per cent. Carbon Steel on Heating and Quenching. 

the remaining free ferrite. Quenching, column IV, will therefore 
produce a " harder " steel than in case III, but nevertheless without 
complete refinement of the steel as a whole. 

Heating to a temperature D, slightly over the upper critical 
range, if prolonged for a length of time sufficient to effect complete 
diffusion and equalization, will entirely refine the steel, giving it the 
smallest grain size possible. Quenching, column V, will retain this 
condition and give the maximum hardness possible. 



HARDENING 161 

Heating to a temperature E, considerably over that of the upper 
critical range, will tend to increase the grain size; and quenching, 
column VI, will retain this condition, giving a more brittle steel. 

Relation of Hardening to Annealing. — Thus it will be seen that 
during the heating operation the changes taking place in the micro- 
scopic constituents and the structure as a whole are similar in both 
hardening and annealing. The main difference in the final results of 
the two processes is due to the rate of cooling through the critical 
ranges, and, therefore, upon the nature of the micro-constituents 
which are thereby retained in the steel when cold. 

The effect of slow cooling through the critical ranges, which is 
characteristic of true annealing, has been discussed; in brief it may 
be said that the austenite or solid solution shifts its carbon content 
through generating pro-eutectoid ferrite (or cementite) to the eutec- 
toid ratio of about 0.85 to 0.9 per cent, carbon, and then transforms 
with increase of volume at Arl into pearlite, with which the ejected 
ferrite (or cementite) remains mixed. This change or decomposition 
of the austenite, however, does not take place suddenly or spas- 
modically, but develops by stages; and that these intermediary 
stages between austenite and its final constituents may be recognized 
and identified under the microscope as martensite, troostite, osmond- 
ite and sorbite is generally accepted. Hardening is but the result 
of obstructing this transition, thereby retaining in the steel the 
" hard " austenite or its early decomposition products martensite 
or troostite. 

Austenite. — Austenite is only obtained with difficulty in the 
ordinary carbon steels, and even then is usually decomposed in part 
into martensite. The two agents 1 — rapid cooling and carbon — 
tending to obstruct this transition must be grouped in suitable pro- 
portions — that is, the carbon content must be high, and the cooling 
take place with extreme rapidity. With about 1.5 per cent, carbon 
steel, such as generally is used in corrugating and roll-turning tools, 
when quenched in brine or very cold water from about 1400° F., 
about one-half of the austenite will remain unaltered. When the 
carbon is about 1.1 per cent. — which may be regarded as about 
the minimum limit, although some austenite has been obtained with 
water-quenched 0.9 per cent, carbon steel — the cooling must be done 
in iced solutions from a temperature of 1800° F. or more. 

1 Alloys are also obstructing agents in the sense that, if present in the 
proper amount, they lower the temperature at which the transition will com- 
mence. 



162 STEEL AND ITS HEAT TREATMENT 

The hardness of austenite, as preserved in hardened high-carbon 
steels, does not fall very far short of that of the accompanying mar- 
tensite, probably because the austenite is partly transformed into 
martensite in cooling. On the whole, however, austenite may be 
regarded as being considerably softer than martensite, and also much 
tougher; the austenite as obtained in high manganese and high nickel 
steels is but moderately hard. 

Martensite. — Martensite is the chief characteristic constituent 
of hardened carbon steels when cooled rapidly in water from a tem- 
perature above the A3 range. In very high-carbon steels, rapidly 




Fig. 139.— Martensite. X75. (Ordnance Dept.) 

cooled, the martensite is associated with austenite. In the lower 
carbon steels hardened in water, in high-carbon steels hardened in 
oil, or in thick pieces of high-carbon steel hardened in water, mar- 
tensite is associated with troostite and with some pro-eutectoid 
ferrite or cementite. 

Of the transition constituents — austenite to pearlite — martensite 
is the hardest and also the most brittle, having extremely high tensile 
strength with little or no ductility. Microscopically martensite is 
characterized by a needle-like structure as is shown in Fig. 139. 

Troostite. — Troostite is obtained by cooling through the trans- 
formation range at an intermediate rate, as in small pieces of steel 



HARDENING 



163 



when quenched in oil, or quenched in water from the middle of the 
transformation range, or in the center of larger pieces quenched in 
water from above the critical range. The early appearance of troos- 
tite in tool steel is shown in Fig. 140. 

The hardness of troostite is intermediate between that of the 
martensitic and pearlitic state corresponding to the carbon content 
of the specimen. In general, the hardness increases, the elastic limit 
rises, and the ductility decreases, as the carbon content increases. 




*;y Jam. 'A *M 










cAi 



Fig. 140.— Troostite (Dark) in Hardened Carbon Tool Steel. X 100. (Bullens.) 

Sorbite. — Sorbite, when obtained by hardening, is ill defined and 
almost amorphous; it is softer than troostite for a given carbon 
content. Dependent upon the carbon content, sorbite may be 
obtained by quenching small pieces of steel in oil or in molten 
lead, or even by air-cooling them; or it may be obtained by quench- 
ing in water from just above the bottom of the Arl range. Sorbite, 
and to some extent, troostite, are more characteristic of tempered 



164 



STEEL AND ITS HEAT TREATMENT 



steels than of hardened steels. The transformation of troostite into 
troosto-sorbite is shown in Fig. 141. 

Temperature for Hardening. — As a general rule, hardening is 
carried out from a temperature of about 50° F. above the line A3-A2.3 
-Al.2.3 in Fig. 75. This is done to obtain the best refinement 
of the steel as well as the maximum hardening effect. Rapid cooling 
of medium and low-carbon steels from a temperature just above the 




Fig. 141.— Trooste-Sorbite (Dark). X100. (Bullens.) 

bottom of the critical range Al, will not bring out the maximum 
hardening effect. The general temperatures most applicable for 
individual steels are given in subsequent chapters, i 

When the maximum results, both as affecting the structure and 
also the physical results, are to be obtained, experiments should be 
made to determine exactly how far over the critical range the steel 
should be taken. In some steels it will be found that approximately 
50° F. will accomplish this purpose; in others it may be necessary 
to raise the temperature to even 150° F. or more. This effect will 



HARDENING 



165 



also be influenced by the size of the piece to be hardened, as will 
be shown under " Tool Steel." 

Heating for Hardening. — The general rules for heating for 
hardening may be simply stated, but their fullest comprehension and 
application may be obtained only in the light of experience. This 
heating requires much more care than heating for annealing (if such 
be possible) , on account of the diametrically opposite functions which 
are indicative of the two operations. Heating for annealing is 
followed by slow cooling and the gradual release of all stresses 
and strains: heating for hardening is followed by the most severe 
test to which steel can be put — very rapid cooling, accompanied 
by the setting up of a condition of stress and strain. In general, 
we may say that the heating for hardening should be slow, uniform, 
and thorough, and to the lowest temperature which will give the 
desired results. 

The same elements of temperature-time-mass-surface and initial 
structure affect the heating and cooling phases of hardening just as 

Effect of Initial Structure on Hardening. 





Fig. 142. — First Treatment. 
(Bullens.) 



X100. 



Fig. 143. — Second Treatment. 
(Bullens.) 



X100. 



much as of annealing. When the initial structure shows a coarse 
^rain size and overheating, the factors of diffusion and equalization 



166 STEEL AND ITS HEAT TREATMENT 

Effect of Initial Structure on Hardening. 




Fig. 144— Third Treatment. X10G. (Bullous.) 




Fig. 145. — Fourth Treatment. X100. (Bullens.) 

become paramount, so that prolonged saturation, or repeated harden- 
ing, must be used. This is illustrated by the photomicrographs of 



HARDENING 167 

Figs. 142, 143, 144 and 145, which represent the effect of repeated 
quenchings (and toughening) of a 10-in. diameter shaft, 0.4 per cent, 
carbon, having an initial structure similar to that of Fig. 125. It will 
be noted that it required four quenchings to entirely sorbitize the 
steel. The effect of initial structure is likewise shown by Figs. 54 
to 59 and the discussion on pages 82-85. 

Non-uniformity in heating must of necessity result in lack of 
uniformity in cooling, which in turn is the genesis of most of the 
troubles in the hardening process. Hardening cracks are more 
often the result of uneven heating than of a defect in the steel. 
Heating requires time and care. The peculiarities of each steel and 
article must be thoughtfully studied; experiments must be made; 
and the clear judgment of experience applied to each individual 
case. It has been well said that " Steel is mercurial and delicately 
responsive to heat; its records appear in its own structure." 

Lowest Quenching Temperature the Best. — The lowest heat 
which will give the results desired should always be used in hardening. 
This point can be brought home in no better way than to give the 
results of two tests made which illustrate exactly this principle. 
Two automobile gears were made from the same bar, by the same 
man, and in all other ways as nearly alike as possible. The tests 
were made by a disinterested third party. 

Number 1 was quenched in oil from 1450° F., annealed at 1400°, 
hardened in oil from 1450°, and tempered at 475° in oil. It gave a 
sclerescope hardness of 76 to 78. It withstood 48 blows of a 10-lb. 
hammer dropping 30 ins. before a tooth could be broken out, or 8 
blows of a 10-lb. hammer dropping 48 ins. 

Number 2 was quenched in oil from 1400°, which was just 
over the critical range, and determined by when the magnet " let 
go." It was annealed at just under that temperature, followed 
by hardening in oil from 1400° and tempering in oil at 475°. The 
hardness was the same as with No. 1. In this case, however, it 
required 200 blows of the 10-lb. hammer falling 30 ins., or 78 blows 
with a fall of 48 ins. 

The effect of the increase of only 50° in the hardening temperature 
is self-evident: it meant a difference in efficiency in the ratio of 48 
to 200. 

Temperature of Quenching. — It is not only the uniformity of 
heating of the steel object which is necessary for uniform and proper 
hardening, but also — and equally important — the uniformity of 
temperature of the piece at the moment of quenching. A piece of 



168 



STEEL AND ITS HEAT TREATMENT 



steel may be heated properly at the moment previous to its with- 
drawal from the furnace; but that same piece may have wide dif- 
ferences of temperatures in different parts of the mass at the moment 
of quenching. Non-uniformity of heat saturation at the latter 
instant must inevitably result in non-uniformity of hardening — 



F. 

1800 

1700 
1600 
1300 


















































































1400 
























1300 

1200 























































0.2 



0.4 



0.6 



0.8 



1.0 



1.2 



IA$ C 



Fig. 146. — Carbon-iron Diagram Showing Temperatures at which Ordinary 
Carbon Steel Loses Its Magnetism on Heating. 

with the attendant possibility of warping, cracking and similar 
features. Whether or not the indirect cause of such i a condition is 
due to the shape and size of the object or to the method of hand- 
ling the stock between the furnace and bath is immaterial as far as 
the basic principle outlined above is concerned. These are but a 
manifestation of the ever-present " human element." 

Overheating. — Overheating is probably one of the most com- 
mon sins of the hardening shop. Unfortunately, many " practical " 



HARDENING 169 

men still believe in the efficacy of high temperatures for greater 
hardening effect. Although this may — to a very limited extent — 
be true, the weakening of the steel by the increase in grain size and 
greater hardening strains as obtained by high temperatures more 
than offset the questionable production of greater hardness. Fully 
80 per cent, of the complaints of " bad steel " are the direct result of 
overheating. Both theory and practice support the old rule that 
" the lowest heat which will give the desired results is the best heat." 
The Magnet in Hardening. — It will be recalled that steel becomes 
non-magnetic (for all practical purposes) in passing through and 
beyond those temperatures represented by the heavy black line in 
Fig. 146. For steels with about 0.35 per cent, carbon and upwards 
this temperature line also corresponds to the best refinement of the 
steel in heating. We, therefore, have a very simple and practical 
means of determining the proper temperatures for hardening such 



Fig. 147. — A Magnet Used in Hardening. 

steels. All that is necessary is to apply an ordinary horse-shoe 
magnet, suspended from a suitable rod, against the hot steel. When 
the correct hardening temperature has been reached there will be 
no attraction between the magnet and the steel. For steels with less 
than about 0.3 per cent, carbon the rise in temperature between the 
Ac2 and Ac3 ranges may be estimated and the steel hardened when 
it has reached the latter temperature. It will be found that the use 
of the magnet will be of great value to those not having the proper 
pyrometer control over their heating operations and who have to 
depend entirely upon the eye for gauging such. An instrument 
more convenient for this purpose than the ordinary magnet is made 
by magnetizing a small, elongated, diamond-shaped piece of steel, 
and supported between two pins in the end of a forked rod, as is 
shown in Fig. 147. 

Motion during Hardening. — When a piece of steel is quenched, 
movement should be given to the steel, to the quenching medium, 
or to both. This is for two reasons: (1) in order to cool the steel as 
rapidly as possible, and (2) to break up any tendency toward the 
formation of a distinct line between the hardened and unhardened 
parts of a differentially hardened forging. The first factor should be 
self-evident: agitation of the bath or movement of the hot steel 
will lower the temperature of the oil or water which is cooling the 



170 STEEL AND ITS HEAT TREATMENT 

steel will prevent the formation of vapor or steam around the 
steel, and in other ways more rapidly cool the metal. In the 
second place, if a piece of hard steel, such as a chisel or die- 
block, were to be immersed to a certain point and held there 
quietly, the rapid cooling would harden the steel up to the point 
of immersion and no further; in other words, there would be a 
sharp line of demarkation between the hardened and unhardened 
parts, and which in turn would be a source of great weakness and 
possible fracture. So, in quenching, move the piece up and down 
in the bath; or if it is to be only partly immersed, agitate the 
bath so that there will be no distinct line of hardening: avoid 
straight-line hardening. 

Heating Baths. — Despite the efficiency of design of many 
furnaces, — and not mentioning those of poor design — their often 
inefficient operation has a general tendency towards non-uniformity 
in heating and oxidation. In the effort to solve these two problems 
at once — that is, to surround the object to be heated with a con- 
stant and uniform heat on all sides, and to avoid contact with the 
air — the application of various molten baths has come about. 
Chief of these heating mediums are molten lead and certain salts. 
On account of the operating cost and necessarily small capacity, 
however, their use is largely confined to the heating of tools and 
other articles requiring particular care, uniformity, and freedom 
from oxidation. The principal use for such baths should be for 
the retention of a bright surface on the metal after hardening, 
and not for uniformity in heating; any furnace, properly designed 
for the work in hand, heated with the right fuel, and correctly 
operated should give entire uniformity of heating. There is no 
difference between heating steel in a bath of gases and heating 
in a bath of liquid, if the piece to be heated is exposed in each 
case in the same manner, for the same time, at the same tem- 
perature, although there may be a difference in the finish or surface 
appearance. 

Heating in Lead. — Before the advent of the modern heat treat- 
ment furnace, heating in molten lead represented the most practical 
method of obtaining uniform heating. With a reasonable amount 
of care and attention the typical lead bath may be maintained at 
such temperatures as the ordinary hardening operation requires and 
with a satisfactory degree of uniformity. Its use, however, presents 
many difficulties. The bath must be frequently agitated to preserve 
a uniform temperature. When heated to over 1200° F. lead begins 



HARDENING 171 

to volatilize, giving off fumes which are both offensive and poisonous; 
suitable ventilation, such as may be obtained with a properly de- 
signed-hood, should be provided to remove these fumes. Further, 
the bath must be covered with powdered charcoal to reduce the oxides 
or dross which are formed in the molten lead. Many plants will not 
use lead baths if temperatures greater than 1475° or 1500° F. are 
necessary. On account of its high specific gravity, heating in lead 
requires some method of holding the steel beneath the surface, as 
otherwise the tool would float on the surface of the bath and thus be 
unevenly heated. One of the most troublesome difficulties with 
lead baths is the tendency of the lead to stick in the holes, threads, 
or even to the surface of the tool when it is removed for quenching, 
so that uniformity of cooling is sometimes materially affected. Al- 
though this particular difficulty has been largely eliminated by the 
use of a paste, the trouble may simply be aggravated in case this 
coating has not been carefully and properly applied. 

Salt Baths. — Many of the difficulties encountered in the use of 
lead for heating may be overcome by the substitution of different 
salts. Their lower specific gravity permits of a more uniform 
circulation and there is no tendency of the tool to float on the sur- 
face of the bath. At the usual temperatures used for hardening 
there is little or no vaporization. Although lead may prevent the 
steel from oxidation while the steel is being heated, as soon as the 
tool comes in contact with the air on removal from the lead bath, 
a thin film of oxide is formed; with the salt bath, on the contrary, 
the steel receives a thin and uniform coating of molten salt, which 
protects the surface of the metal. 

The minimum temperature of the salt bath may be very closely 
estimated without the use of a pyrometer. Common table salt has 
a freezing-point of 1472° F., and if it should be melted with potassium 
chloride (freezing-point 1325° F.) or other salts, the freezing-point 
of the melt may be quite accurately adjusted over a wide range of 
temperatures. By keeping the bath very near its freezing-point by 
a suitable regulation of the heat, overheating of the steel may be 
entirely overcome. Further, if the composition of the salt bath has 
been so adjusted as to approximate the proper hardening temper- 
ature, when the steel is removed from the bath it may be quenched 
just at the time when the salt film begins to solidify — or at exactly 
the correct temperature. 



172 STEEL AND ITS HEAT TREATMENT 



BATHS FOR QUENCHING 

General Properties of Quenching Media. — The main thought in 
selecting a proper bath for quenching is the rapidity with which the 
heat is removed from the hot steel. This property of transference or 
withdrawal of heat from the solid by and to the liquid, will depend 
upon the specific heat of the liquid, its conductivity, viscosity and 
volatility. That is, the specific heat will indicate the heat-absorptive 
power of the liquid; the conductivity will measure its capacity for 
transferring the heat thus absorbed to the cooler part of the bath; 
the viscosity affects the motion of the liquid and thus influences the 
uniformity of cooling; and the volatility indicates the temperature 
at which the liquid will become gaseous, thus forming a vapor around 
the steel and preventing the quick removal of the heat from the 
steel. By obtaining a suitable combination of these various prop- 
erties a bath giving the desired effect may be obtained. 

Temperature of the Bath. — The continuous use of any bath for 
quenching will gradually and progressively raise the temperature of 
the liquid used. As a general rule, differences in the temperature of 
the bath will give rise to varying results in the actual hardening taking 
place— the higher the temperature of the bath, the less its cooling 
efficiency. This is especially noticeable with water; a change of 
50° or 100° F. will often entirely alter the physical properties of the 
quenched steel. The effect is less marked with oils, and with some 
oils may be almost negligible for certain classes of work. On the 
whole it is decidedly better practice to maintain as nearly as possible 
a standard temperature in the quenching bath. 

Quenching Speed of Different Media. — It is evident that the 
cooling medium used, its temperature and condition will affect the 
rate of cooling. Matthews x and Stagg have devoted considerable 
time to investigating numerous commercial media which are in use 
in typical hardening plants of the country at the present time. 
Their method was as follows : A suitable test piece was machined from 
a solid bar, and a hole drilled through one end to within an equal dis- 
tance from each side and bottom of the test piece. Into this hole 
a calibrated, platinum-rhodium couple was inserted and the leads 
connected to a calibrated galvanometer. The test piece was then 
immersed in a lead pot, and the lead pot was maintained at a tem- 

1 Matthews and Stagg, " Factors in Hardening Tool Steel," A. S. M. E., 
1915. 



HARDENING 173 

perature of 1200° F. When the couple inside the test piece was at 
1200° F., and the couple in the lead pot also read 1200° F., the test 
piece was removed and quenched in 25 gals, of the quenching medium 
under consideration. At the start the quenching medium was at 
room temperature. The time in seconds that it took the test piece 
to fall from a temperature of 1200° F. to a temperature of 700° F., 
was noted by the aid of a stop-watch. It is clear that immersing the 
test piece in the quenching medium raised the temperature of the 
medium. The test piece was then replaced in the lead, heated to 
1200° F M quenched into the medium at this higher temperature and 
the time again taken with the stop-watch. These operations were 
continued until the quenching medium, in the case of oils, had 
attained a temperature of about 250° F. The results obtained, time 
in seconds, for a fall from 1200° F. to 700° F., were plotted against 
the temperature of the quenching medium and a series of curves as 
shown in Fig. 148 l were obtained. 

The various curves represent the following quenching media : 

W. Syracuse city water. 
B. Brine. 

Sec. 

1. New fish oil; average of readings from 80° to 250° F. . 85 

2. No. 2 lard oil 87 

3. Prime lard oil in use two years 99 

4. Boiled linseed oil 101 

5. Raw linseed oil 102 

6. New extra-bleached fish oil 106 

7. New yellow cottonseed oil 107 

8. New tempering oil; 60% cottonseed, 40% mineral. . . . 122.6 

9. New mineral tempering oil 130 

10. No. 1 dark tempering oil 157 . 3 

11. Special "C" oil 164.7 

A consideration of the results is interesting. Pure water (curve 
W) has a fairly constant quenching rate up to a temperature of 100° 
F., where it begins to fall off. At 125° the slope is very marked. 
Brine solutions (curve B) have both a quicker rate of cooling and 
are more effective at higher temperatures than water. The curve 
does not begin to fall off seriously until a temperature in the 
neighborhood of 150° is reached. Where water at 70° cooled the 
test piece in 60 sec, the brine solutions cooled it in 55 sec. 

1 For the sake of brevity and clearness the numerous curves as given by Mat- 
thews and Stagg have here been grouped under one plot. 



174 



STEEL AND ITS HEAT TREATMENT 



As is well known, the oils are slower in their quenching powers 
than water or brine solutions, but the majority of them have a 
much more constant rate of cooling at higher temperatures than water 
or brine. The curves shown in 10 and 11 are for thick viscous oils 
similar to cylinder oils. These curves are particularly interesting 
in that they have slower quenching abilities at low temperatures than 
at higher temperatures. A comparison of curves 2 and 3 shows the 
variation in quenching power of the same oil due to continued ser- 

250 



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E100 



50 









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10 




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50 



100 150 

lime in.Seconds 



200 



Fig. 148. — Quenching Power of Liquids. 



vice. The differences in quenching rates may well account for 
different results from the same steel in different shops, Or in the same 
shop due to change of oil used. 

Water Spray for Hardening. — Water sprayed under pressure is 
the quickest agent for rapid cooling in common use, exceeding in its 
hardening qualities either brine or water baths. The main point to be 
noted is that there shall be sufficient volume and pressure to prevent 
the formation of a blanket of steam between the hot steel and the 



HARDENING 175 

spray. Its most common use is for such tools as sledges and others 
requiring a differential hardening, and for armor plate. 

Brine. — Brine is used only in certain particular lines, such as file- 
hardening, for which an extremely hard surface is required. Unless 
the steel has been most carefully heated, and is of a proper chemical 
composition, quenching in brine is almost certain to crack the steel. 
This is particularly true of large sections, for in these the very sudden 
cooling of the outer surface, while the center is still hot, will set up 
stresses and strains which will not be relieved or equalized in the 
short time allowed, and with the inevitable results. 

Water Quenching. — Water cools the steel more rapidly than oil, 
but its more drastic action increases the internal strain and conse- 
quent liability to fracture. For the low-carbon steels, and for small 
and comparatively simple sections of the higher carbons, water 
quenching may be used without much danger. Of course in cases 
where it is required that the surface shall be glass-hard, or that 
the maximum tensile strength be obtained, water quenching is man- 
datory. On the other hand, if the steel is to be given a full heat 
treatment (i.e., quenching and toughening), the difference in hard- 
ness as obtained by the two baths may usually be nearly equalized 
by using a lower drawing temperature for the oil-quenched piece; 
that is, if a 0.40 per cent, carbon steel forging is quenched in water 
and toughened at say 1200° F., approximately the same static prop- 
erties may be obtained by oil quenching and a subsequent reheating 
to say 1050° F. The principal objections to the latter method are 
that the lower drawing temperature is not so easily recognized by its 
color, nor will the dynamic properties probably be quite as high — 
though this last point is questionable. Generally speaking, however, 
oil quenching is more desirable than water quenching. 

Oil Tempering. — The term " oil tempering/' referring to the 
quenching in oil, is one which has become current in the trade, so 
that the term, " hardening " often refers to quenching in water only, 
or in some medium which will give an equivalent or greater hardness. 
Strictly speaking, the use of " tempering " in this sense is a mis- 
nomer, for it should be used as indicative of a slight reheating or 
" softening " of the quenched steel. 

Special Quenching Methods. — It often happens that especially 
high tensile results are desired in certain large forgings of such size 
and chemical composition that direct quenching in water is deemed 
unwise, and yet in which it is desired to obtain as near the maximum 
effect of water cooling as possible. A method which has proven in a 



176 STEEL AND ITS HEAT TREATMENT 

large measure successful is to use a bath of oil resting upon an equiva- 
lent or greater volume of cold water. The forgings, when heated 
to the proper temperature, are lowered into the oil for a few seconds 
and thence into the water. The oil forms a film on the surface of the 
steel, so that the sudden effect of the water is somewhat diminished 
or retarded. The rapidity of cooling may be controlled by the dura- 
tion of the oil quenching. It is obvious that in using this method 
there must be a sufficient volume of water under the oil to prevent 
the formation of steam and its consequent danger. 

For small tools or thin instruments such as saws, the above 
method may be so modified as simply to have a film of oil upon the 
surface of the water, the oil in this case consisting of some animal 
or vegetable oil. The heated tool is plunged directly and evenly 
through the oil film so that it enters the water with a thin coating 
of burnt oil which protects it from the direct action of the water and 
lessens the risk of fracture. The amount of oil may of course be 
increased as desired. The main objection to these methods is the 
lack of uniformity in hardening unless the operator has had more or 
less experience. 

A method which is extensively used in some tool works is that of 
using a combination of water and oil quenching, that is, first plunging 
the tool into water until a certain amount of heat has been removed, 
and then transfer to the oil, where it remains until cold. 

Molten lead is sometimes used as a quenching medium for small 
sections in which great toughness and only a moderate degree of hard- 
ness is desired. Although dependent upon the carbon content, steel 
subjected to this process will generally be sorbitic Such treatment 
will require no further reheating. 

Other Aqueous Quenching Media. — Hardeners, at one time or 
another, have tried about everything under the sun in the attempt 
to discover some new and wonderful quenching medium which would 
accomplish the phenomenal. The results, for the most part, do not 
warrant the addition of expensive chemicals; and if the experi- 
menters do claim the marvelous, the " gold-brick " scheme is gen- 
erally revealed by thorough investigation. 

Some substances, such as lime, soap, etc., may be added to form 
a protective coating around the steel. Calcium chloride will raise 
the boiling-point of water to a considerable degree, so that the solu- 
tion may be used at a temperature up to 150° or 175° F. without 
danger, and at the same time give many of the advantages which 
oil hardening possesses. Some salts increase the hardening effect of 



HARDENING 177 

water; others purify the water or soften it. One of the most inter- 
esting (and wonderful?) combinations noted contained — by addition 
— ammonia, glycerine, sal-ammoniac, spirits of nitre, ammonium 
sulphate, alum and zinc sulphate! 

Differential Hardening. — In certain tools, such as anvil faces, 
die blocks, edge tools, and the like, it is desired to obtain a very hard 
outer part, surface or edge, to be " backed " by a less hard and 
tougher steel. That is, the steel is gradually and progressively to 
change from extreme hardness to the opposite, or what we may 
term differential hardening. This phase may be obtained either 
by heating the whole mass of the steel — as in die blocks, or by heating 
only part of the article — as in chisels; in either case that part which 
is to have the greatest hardness is immersed or quenched. By this 
method the heat is gradually withdrawn from the part not immersed 
through that part which is being subjected to the cooling bath, so 
that the mass of steel as a whole will become progressively softer 
or tougher from the hardened face or edge to the opposite side. 
Precautions must be taken to avoid straight-line hardening, 

Cooling the Water Bath. — Where water is used as the quenching 
medium it is customary to maintain a flow of fresh, cold water into 
the quenching tank so as to keep a uniform temperature and purity. 
Water which has been used for any length of time without renewal 
goes " stale " with a corresponding loss in cooling efficiency. If the 
cost of water is such that it is inadvisable to dispose of the overflow 
from the tank, the hot water may be cooled by spraying, cooling 
towers, etc., aerated, and then returned to the tank. 

Cooling the Oil Bath. — The common methods for cooling the oil- 
quenching bath may be broadly classified as follows: (1) The cir- 
culation of cold water around, or through coils in the bath; (2) the 
circulation of the oil itself; (3) by the use of compressed air. 

One of the simplest methods for cooling the oil when in small 
tanks and not too constantly used, is to place the oil tank within a 
larger tank, with a space of say 2 to 6 ins. between the two tanks. 
This space is kept filled with cold water. As in all these systems, 
the intake should be at the bottom of the tank, with the outlet or 
overflow at the top. The main objection to this method is the fact 
that the heat in the oil must penetrate through the walls of the tank 
before it can be conducted away by the water. 

The next type of cooling makes use of coils or radiators placed 
within the oil tank and the circulation of cold water through these 
pipes. These water lines are placed close against the side of the 



178 



STEEL AND ITS HEAT TREATMENT 



tank so that they may not interfere with the work being treated. 
The radiator type as shown in Fig. 149 does not give as great effi- 
ciency as the simple coil system in Fig. 150. With the difference of 



^^ 



C^ 



n 



n 



n 



32 



Fig. 149. — Radiator Type of Cooling System. 

temperature of the oil in the bottom of the tank, as contrasted with 
the hotter oil at the top, it is difficult to obtain a thorough circulation 
of the cold water through all sections of the radiator. Further, this 




fl 



C^ 



rr 



HT 



rr 



n 



5) 



Fig. 150. — Coil Type of Cooling System. 



same difference in temperature has the tendency towards unequal 
expansion of the top and bottom pipes, which may cause a leakage 
of water into the oil and its attendant dangers. In the coil system 
there is of necessity a complete circulation, together with the elim- 
ination of expansion dangers. These pipes vary in size from about 



HARDENING 179 

1| ins. to 3 ins. diameter; the latter size has given excellent satis- 
faction in a tank approximately 8 ft. wide by 16 ft. long and with a 
working capacity of about 8000 gals, of oil. Guide strips should be 
placed at intervals along the coils — from top to bottom — to prevent 
any articles from catching against the pipes while the quenched 
material is being raised out of the tank. 

Circulation and Cooling of the Oil Itself. — The best results for 
keeping down the temperature of the oil bath are undoubtedly, to be 
had when the oil itself is circulated. The circulation is continually 
bringing cold oil into the vicinity of the hot metal, removing the hot 
oil from the tank, as well as giving a more uniform temperature to 
the bath as a whole. In the previous systems the heat must be 
taken away by gradual and progressive transference from the region 
of the hot steel towards the sides of the tank, and at the best is a 
slow procedure — this is assuming that the oil is not kept in motion 
by compressed air. In the present system, the heat is taken away 
from the quenching bath by the actual removal of the hot oil 
itself. 

The usual methods are to pump the hot oil from the tank and then 
through coils which are cooled by suitable means; or by maintaining 
large supply tanks in which the oil will have sufficient time to cool 
before being returned to the quenching tank. In the former pro- 
cedure the coils containing the hot oil may be cooled by refrigerating 
— such as the ammonia process, etc. — or by placing the coils in a 
water tank, or by cooling the coils with a continual stream or spray of 
water. Where the size of plant will permit the installation of a 
refrigerating system, such a method is by far the most satisfactory; 
the heat may be removed very quickly, and the temperature of the 
oil controlled at any desired temperature by the regulation of its 
flow through the cooling coils. 

As an example of the water-bath method, one steel company 
pumps the oil from the quenching tank — holding some 12,000 gals. — 
through 3-in. pipes and thence through coils placed in a large water 
tank used for the mill supply. The cold oil then returns to the 
quenching tank by gravity. 

For smaller plants the coils may be most conveniently cooled 
by the use of tiny streams of water trickling over the coils. On the 
whole, this is probably the most satisfactory system of all for small 
plants. In one case (in which the question of the cost of water 
was important) this method was found to be both cheaper and to give 
a higher cooling efficiency than could be obtained by setting the coils 



180 STEEL AND ITS HEAT TREATMENT 

in a small water tank. In the latter case the heat is removed by 
transference from one part of the water to that further removed 
from the coils, so that unless a very good flow is maintained, the 
cooling will be comparatively slow. Further, the water removed 
from the tank is, on the whole, but lukewarm, and therefore but 
imperfectly accomplishes its mission. On the other hand, in the 
drip system a small amount of cold water is always in contact with 
the coil, giving a maximum cooling efficiency with a minimum 
expense. 

A recent heat treatment installation l attacks the problem of 
keeping the quenching medium at a uniform and low temperature by 
the maintenance of a large and separate supply of oil. The hardening 
is done in special quenching tank cars, as shown in Fig. 151, and 
which are wheeled to any furnace desired. Just before quenching 
commences the valve in pipe K is turned on and a 2-in. stream of cold 
oil is kept flowing into the tank. The hot oil passes out through the 
overflow pipe L, through the hole in the floor and into a pipe that 
conducts it into an underground tank. This underground pipe is 
made very large, so that there will be no danger of its clogging, 
which would necessitate tearing up the floor. Each furnace through- 
out the 400-ft. length of the shop is provided with a similar inlet 
pipe and floor hole connection to the pipe which carries away the 
overflow. From the underground tank the oil is pumped to 
upright tanks close to the outside of the building; from these 
tanks the oil flows by gravity to the tank cars. 

Use of Compressed Air.— The advisability of using compressed 
air in the quenching tank is a much debated point. If applied 
intelligently, however, it undoubtedly renders great assistance in 
the hardening and cooling operations. In systems in which the oil 
is kept in constant and fairly rapid circulation, it is neither required 
nor advised. 2 But if the oil is cooled by the circulation of water 
in pipes, the use of compressed air is often mandatory in order to 
obtain the maximum, as well as uniform, cooling efficiency of both 
oil and water. In any case, the air must not be allowed to come in 
contact with the hot steel, as soft spots would result; neither should it 
be used in too great quantities nor pressure, especially with the 
heavier and low-grade oils, as it may cause the precipitation of 

1 " A Modern Heat-Treatment Plant," E. F. Lake, Machinery, Sept., 1914. 

2 The cold oil forced into the quenching tank may be distributed under pres- 
sure to different parts of the tank, thus providing excellent circulation, and 
accomplishing the same results as compressed air. 



HARDENING 



181 



certain constituents of the oil, or cause the formation of a scum or 
foam on the surface of the oil. When the air sets up a fairly efficient 




St 

O 0! 



pj 



3 



s 



circulation of the oil (or water, if water is the quenching bath), it 
has accomplished its mission. Compressed air should rarely be 
used with animal or vegetable oils on account of oxidation. 



182 STEEL AND ITS HEAT TREATMENT 

Size of Quenching Tank. — The volume of the quenching medium 
to be used, and hence the size of the tank, depends principally upon 
the size and number of the pieces to be hardened, and also upon the 
method used for cooling the quenching bath. The tank should 
always be of sufficient size to take with ease the maximum size stock 
to be treated, besides a generous allowance on all sides for a suffi- 
cient body of oil or water, for rapidity in handling the material, and 
for circulation. Further, the size of the tank should be proportioned 
to the degree to which the solution can be kept cooled when the hard- 
ening department is operating at maximum capacity; the more 
efficient the cooling system, the smaller the size of tank necessary. 
On the whole, it is decidedly preferable to have the tank too large 
than too small. 

CRACKING AND WARPING 

Influence of Non-uniformity of Section on Cracking. — One of the 
main causes of steel breaking in hardening is from the unequal con- 
traction and expansion in different parts of the steel. If it were pos- 
sible to get every particle of the steel cold at the same moment there 
would be an end to danger of this sort. But as this is a physical 
impossibility, we must approach such a condition as near as we can. 
This danger of cracking is particularly emphasized in forgings or 
tools of unequal thickness. If the thinner part should be first 
immersed in the quenching bath (e.g., water), it would become cool 
much sooner than the heavier sections; that is, the thin part would 
be cold or " fixed " while the thicker part of the article was still 
contracting from loss of heat. Hence the thin part in its then hard 
and brittle state cannot " give " and will consequently break; or, 
if it does not break at the time of hardening, the steel is held in such 
a state of stress that it is ready to break when applied to the work, 
or even when being tempered. These influences are the more marked 
with the greater the rapidity of cooling and hardening effect of the 
bath, as well as with the increase in carbon content and alloys. 

Influence of Bulk of Section on Cracking. — Further, the danger of 
cracking is dependent upon the bulk of the article, even though it 
be of uniform section. Its effect is repeatedly illustrated by large 
forgings such as locomotive axles, crank-pins, etc., of rather high 
carbons quenched in water. This point is illustrated by the case 
of a locomotive crank-pin which had been hardened in water and 
then toughened. A thorough examination of the forging before 
shipment to the railroad company revealed no external evidences 



HARDENING 183 

of any crack; but when it had been in service but a very short time 
it fractured badly. Examination then showed that it had evidently 
been in a state of stress within its center, with the development of 
an embryo crack; the dynamic stresses to which it had been sub- 
jected in service were sufficient to raise the tension beyond what the 
steel would stand, with the resultant internal fracture and its pro- 
gressive development into complete rupture. 

Expansion and Contraction. — In view of recent research work 
this phenomenon of cracking may be explained in a theoretical 
manner along the following lines. We know that when a piece 
of steel is heated through the critical range the formation of 
austenite takes place with a decrease in volume; and a somewhat 
corresponding and opposite increase in volume occurs when it is 
cooled through the same critical range. Now if a large forging of 
considerable diameter is quenched rapidly, the outer sections will 
be held in the hardened condition, and therefore rigid and stressed. 
Meanwhile the interior of the steel, being cooled much less rapidly, 
will in all probability actually pass from the austenitic-martensitic 
condition into that of pearlite, accompanied by the increase of 
volume noted above. If the outer portion or surface of the steel 
is unable to withstand this expansive force, rupture must necessarily 
occur. Illustrative of this, heavy locomotive axle forgings, after 
removal from ^he oil-hardening bath, actually have burst open with 
a tremendous report. However, if the forging has not been hardened 
too drastically, and is removed from the quenching bath before 
entirely cold, an immediate reheating or toughening process will 
generally relieve these stresses before any actual damage takes 
place. 

Hollow Boring. — In order to avoid such dangers, there appears to 
be a decided tendency toward requiring the drilling of axles, shafts 
and heavy forgings of large diameters to provide for heat treatment 
and to remove defective material. It is undoubtedly the fact that 
heat treatment will not attain its full effects in the core of a large 
section. With a solid axle, the heat, upon quenching, is removed 
by a flowing from the center to the outside and thence to the hard- 
ening bath; the amount of heat is so great, however, that at the best 
the core will be but semi-hardened, and in most cases will but be 
grain-refined, or annealed. This point was well illustrated by one 
company in its experiments: it split open a large, heat-treated 
driving axle; the fracture showed that the heat treatment had 
penetrated the ends to a depth of about 6 or 8 ins., and on the 



184 



STEEL AND ITS HEAT TREATMENT 



sides to a depth of about one-half the radius; the fracture of the 
core was similar to that of annealed steel. Again, the loss of duc- 
tility and failure of the heat-treatment process thoroughly to pene- 
trate the core of semi-hardened steel are shown by the following 
results obtained from a 12-in. axle, heat treated, and taken at regular 
intervals from the center to the outside : 





Tensile Strength. 


Elastic Limit, 


Elongation, 


Reduction 




Lbs. per Sq. In. 


Lbs. per Sq. In. 


per Cent in 2 Ins. 


of Area, per Cent. 


1. (Center) 


95,000 


60,000 


7.5 


9.6 


2. 


99,750 


60,000 


15.0 


35.3 


3. 


104,500 


65,000 


17.5 


35.7 


4. 


104,500 


65,500 


19.0 


40.3 


5. (Outside) 


106,500 


70,000 


21.5 


47.7 



Treatment: Quenched in water from 1580° F.; toughened at 1100° F. 
Analysis: Carbon, 0.35; manganese, 0.56; phosphorus, 0.020; sulphur, 0.024; 
1.19; chrome, 0.31. 



nickel, 



By means of drilling a hole through the axle, the quenching solu- 
tion is able to remove the heat from both the inner and outer part 
of the axle at the same time. Hollow-bored axles should be quenched 
vertically whenever possible, and a constant flow of the oil or water 
through the bore be supplied. 

The American Railway Master Mechanics' Association in its 
proposed specifications for alloy steel locomotive forgings (June, 1914) 
calls for " drilling forgings over 7 ins. in diameter, unless otherwise 
specified by the purchaser. The committee has found a great tend- 
ency among users of quenched and tempered steel to require drilling 
of parts over 7 ins., and this practice is advocated by steel-makers. 
In the case of axles and crank-pins particularly, drilling takes away 
practically nothing from the strength of the part; it removes the 
material from the center where defective material is most likely to 
exist and where it is least subject to the beneficial effects of heat 
treatment, and it allows the forging to adapt itself to expansion and 
contraction due to heating and cooling." 

Warping. — Warping is but another manifestation of the effect 
of unequal contraction and expansion, originating mainly in incorrect 
heating or neglect in the manner of quenching, rather than in the 
more drastic effect of the bath itself. Non-uniform heating must 
inevitably result in warping, for if some parts are hotter than others 
when the steel is quenched, it is evident that the rate of cooling over 
the entire length of the piece cannot be the same. The general 



HARDENING 185 

tendency will be for bars to buckle or twist, due to unequal contrac- 
tion during hardening. Take, for example, a bar which has been 
placed upon the relatively cold floor of a heating furnace in which 
the main heat application comes from above. Under these condi- 
tions the tendency will be for the bar to become more heated along 
the upper surface than in that in contact with the cold floor. If 
the bar should now be quenched, the under part — being lower in 
temperature — would contract first (provided it were heated and 
quenched from a temperature over the critical range) and thus 
become bowed. But if the temperature in the cooler part of the bar 
were under the critical range, the tendency would be to bend in the 
opposite direction. Other variations in heating might give a double 
bend; certain localized heating might even cause twisting or tor- 
sional strains. 

Manner of Quenching. — Uniformity of quenching is requisite 
to good hardening work. As a general rule, objects should be 
quenched vertically in the direction of their greatest length. Like 
all rules, there are certain exceptions which must be made to this 
general statement — such as in the case of half-rounds and articles of 
a corresponding design, as well as in such cases where economic 
handling requires other methods, as with shafts, small axles, plates, 
etc. But where no special facilities have been designed for uniform 
quenching, the above rule will be found worthy of adoption for 
symmetrical sections, and especially with unskilled workmen. 

The reasons for this may be best explained by taking small 
automobile drive-shafts as an example. In pulling the piece out of 
the furnace with the tongs, the tendency is to grasp it nearer the 
end than at the middle; consequently, in the general haste to get the 
steel into the quenching bath as soon as possible, the average work- 
man is very apt to drop or plunge it into the oil or water at an angle — ■ 
that is, one end of the piece strikes the quenching solution before the 
remainder of the steel. Hence, initial hardening strains are set up 
which usually result in a bent shaft when it is removed from the 
tank. It is very difficult, in the space of a second or two, to get 
hold of the bar exactly at the middle and also to lower it into the 
water or oil so that both ends are immersed at the same identical 
moment — which this method of quenching demands. Now if the 
workman was to aim at immersing the piece end foremost, as in Fig. 
152, grasping it near the end (as usual) with his tongs, the weight of 
the shaft would automatically tend to bring the shaft to the normal, 
and the quenching would be more nearly uniform. Axles and 



186 



STEEL AND ITS HEAT TREATMENT 



forgings of a similar nature should be quenched vertically whenever 
possible, as less strains are set up in the axle by this mode of quench- 
ing. Extensive investigations by one locomotive builder would tend 
to show that axles quenched horizontally ('as is customary) develop 
a series of stresses which, when plotted, appear as an oval around the 
axis of the axle instead of as a circle. 




Fig. 152. — -Proper Method of Quenching Small Round Bars. 



Hollow forgings, such as guns, hollow tools, etc.,, should always 
be quenched vertically, so that the quenching medium may have a 
free flow through the bore, and also to prevent the pocketing of any 
steam or vapor which may be formed by the contact of the hot steel 
and the solution. 

Round Sections. — The hardening of round sections without 
cracking or bending, and without undue labor cost, presents a problem 



HARDENING 187 

which has attracted much study. The danger of fracture, especially 
of internal origin — whether actual or potential — is always greatest 
in the circular section. This is largely due to the fact that all the 
stresses and their subsequent strains are grouped symmetrically and 
converge upon the central axis. Both the square bar with its corners, 
and the plate or sheet with its larger surface exposure, can more 
easily yield to the internal stresses and afford relief — either in cooling 
during hardening or in the reheating for tempering or toughening — 
than can the circular section. Further, there is greater danger of 
bending and twisting due to non-uniform cooling in the long, round 
bar than in almost any other common section. As has been noted, 
short lengths of rounds of small diameter should always be quenched 
vertically. But when it comes to the handling of large numbers of 
larger bars, either of greater length or diameter, this method is 
obviously at a disadvantage. Yet if the bars are simply dropped into 
the bath by hand, even if every effort is made to have the axis of the 
bar parallel to the surface of the quenching medium, general unsatis- 
factory results are obtained, due to non-uniform cooling. 

One satisfactory method for quenching such bars is shown in 
Fig. 151, in which automobile shafts are handled. The bars, after 
careful heating, are pulled out with long rods which have a hooked 
end, across the inclined steel fore-hearth, J, whence they drop onto a 
jointed rack in the oil tank and are quenched. By starting the bars 
with their axes parallel to the surface of the oil, they must neces- 
sarily be held in the same relative position as they pass down the 
rack into the oil. The rolling also effects a more uniform cooling 
of the shaft in relation to its central axis. Fig. 153 shows how the 
traveling crane lifts one side of this jointed rack to raise the shafts 
out of the oil and dumps them onto the truck at the side. 

An improvement on this method to give further uniformity in 
cooling, and which has been used on finished shafts with almost the 
entire elimination of bending, is illustrated in principle in Fig. 154. 
The apparatus consists of a number of inclined planes or racks 
(similar to that shown in Fig. 153), made from small bars or old rails 
which are held in position by suitable cross-pieces. The hot shaft 
is started down the first plane and passes into the oil or water; 
thence it drops to the next, and so on until it reaches the bottom, 
and is removed by suitable methods. Notice that the change from 
one plane to the next causes a reversal in the direction of rolling, so 
that" any stresses set up by one plane are practically counteracted 
by the next plane, giving a maximum uniformity in cooling. The 



188 



STEEL AND ITS HEAT TREATMENT 



angle of incline has a great deal to do with the practical working out 
of the procedure, and should be varied according to the diameter of 




K 



3 



a 
'a, 

a 

Q 



the bar, its chemical composition, and the nature of the quenching 
medium. The rate of travel down the incline should not be too 
rapid, but should nevertheless be sufficient to allow the reversing 



HARDENING 



189 



action of the several planes to take its effect before the steel is too 
cold. The angle of the planes may be increased as greater depth in 
the solution is reached. The bar should be cold when it reaches the 
bottom of the tank. The angle of the first incline is the most 
important, and should be determined by experiment; it will gen- 
erally be in the vicinity of 10° or 15°. In one plant in which this 
method was used the number of shafts requiring straightening was 
reduced from a very high percentage to less than 1 per cent, of 
the total number treated. 




Fig. 154. — Rough Sketch of Inclined Racks for Quenching Rounds. 



Double Quenching. — The effect of a double quench is, as a general 
rule, to raise the elastic limit and tensile strength without diminishing 
the ductility. This is for the most part due to the higher degree of 
refinement which this double quenching makes possible, thus putting 
the steel in the best possible condition. If the steel is in good con- 
dition (i.e., refinement) before the first quenching, the influence of 
the second quenching will be the less in proportion. It is often cus- 
tomary first to quench from a temperature 100° or 200° F. over the 
critical range, and then, for the second quenching, to heat just enough 
over the critical range to obtain the degree of hardness desired. 



190 STEEL AND ITS HEAT TREATMENT 

For high-carbon steels the double quenching is not to be recom- 
mended except under unusual conditions — such, for example, when 
the steel has been greatly overheated in some previous operation. 
The hardening of high-carbon steels is at best a difficult operation, 
and the less heating to which such steel is subjected the better. 

Manganese on Hardening. — As we have previously mentioned, 
the presence of manganese causes a greater hardening effect, due to 
its obstructing the austenite transition. This increase in hardness — 
in ordinary carbon steels with less than 1.75 per cent, manganese — 
is commonly thought to be associated with an increase in brittle- 
ness, 1 and with the clanger of cracking during or immediately sub- 
sequent to quenching. Forethought must therefore be used in 
obtaining the proper combination of manganese, carbon, and rate of 
cooling to avoid the latter difficulty. The general limits of safety 
for practical work may be broadly (but not invariably) set somewhat 
as follows: water quenching is always dangerous when the mangan- 
ese content runs up around 1.50 per cent., even in low-carbon steels; 
with approximately 1.00 per cent, manganese water quenching may 
be used — although not advised — with mild forging steels; with the 
progressive increase in carbon the manganese content should be 
rapidly lowered, so that in tool steels for water hardening the mangan- 
ese is under 0.40 per cent., and with very high-carbon tools is not over 
0.25 per cent. Dependent upon the size and general shape (design) 
of the piece, as well as the condition (refinement) of the steel, oil 
quenching is generally safe up to 1.75 per cent, manganese with 
0.60 per cent, carbon — in fact, one well-known oil-hardening tool 
steel analyzes about 0.90 per cent, carbon with 1.60 per cent, man- 
ganese. The subject of high manganese steels will be considered 
under a separate chapter. 

1 Refer to Chapter XVII for a further discussion of this point. 



CHAPTER IX 
TEMPERING AND TOUGHENING 

TEMPERING 

Tempering. — When a piece of carbon tool steel is heated to a 
red heat and quenched in water (i.e., hardened), the steel becomes 
hard, brittle, and is held in such a state of stress that its use — 
except in a few particular cases — would be highly inadvisable. 
This hardening operation has arrested the austenitic transition at the 
martensitic stage, and prevented it from advancing further, as into 
troostite, etc. Under these circumstances, the application of heat 
will now accomplish two results: (1) it will relieve the hardening 
strains, and (2) permit the transition to proceed. By properly 
adjusting the temperature of this reheating process, any desired 
stage in the martensite-troostite transition may be obtained. And 
by permitting just the right amount of the hard, brittle martensite 
to go over into the softer and tougher troostite, any desired combina- 
tion of physical properties within the capacity of that particular 
steel may be realized. This process of " letting down " or softening 
is called tempering. 

Troostite. — If the steel has been fully hardened so that it consists 
entirely of martensite, troostite will begin to form at somewhere in 
the vicinity of 400° F., or possibly lower. As the tempering tempera- 
ture is progressively raised, the troostite increases in amount until 
at about 750° F. it begins to change into sorbite. Thus steel in the 
tempered condition is usually characterized by the presence of more 
or less troostite, dependent upon the degree of hardening and upon 
the tempering. Just as martensite may be said to represent the 
condition of hardened steel, or pearlite that of annealed steel, so 
troostite is indicative of a tempered steel — whether it be obtained 
by water quenching and reheating, or by quenching in some less 
drastic medium such as oil but with no reheating. The question of 
whether troostite represents a complete step in the transformation 
is not definitely known, and as far as practical heat-treatment work 

191 



192 



STEEL AND ITS HEAT TREATMENT 



is concerned is but a question of scientific value; the value of troostite 
in its influence upon the hardness and allied properties of tempered 
steel is, however, definitely recognized. 

Hardening Strains. — It always should be remembered that 
tempering not only softens the steel through the influence of troostite, 
but also relieves the strains set up in hardening. This last factor 
should not be lost sight of, for although the proper degree of hard- 
ness is requisite for specific work, no tool will eventually prove of 
much value if it retains the state of strain occasioned by rapid 
cooling. This statement applies not only to water quenching, but 
also to oil quenching (or oil tempering) . Even the influence of boil- 
ing water is often sufficient to relieve more or less of these strains, 
if it is not desired to further soften the steel by higher reheating. 
Naturally, however, the higher the softening temperature the better 
will be the condition of the steel in this regard. 

Temper Colors.- — Nature has provided a useful and more or less 
empirical indication of the degree to which tempering has affected 
the steel through the formation of a surface film of oxide colors (oxide 
of iron). If a piece of hardened steel is brightened with emery 
paper or other suitable means, and then is heated slowly with expo- 
sure to the air, the brightened surface will take on characteristic 
" temper colors." These commence with a very faint yellow and 
progressively change with increase of temperature through varying 
degrees of yellow, brown, purple and blue. That these colors bear 
a definite relation to, and are closely indicative of, a known tempera- 
ture, under certain conditions, is now a generally accepted fact. 
Although a difference in distinguishing the various shades of color 
is bound to occur on account of the " personal equation," the follow- 
ing table is fairly representative: 



Temper- 
ature, De- 
grees 
Fahr. 


Color. 


Temper- 
ature, De- 
grees 
Fahr. 


Color. 


420 


Very faint yellow 


510 


Brown 


430 


Yellowish-white or light straw 


520 


Brown purple (peacock) 


440 


Light yellow 


530 


Light purple 


450 


Pale yellow straw 


540 


Purple 


460 


Straw 


550 


Dark purple 


470 


Dark Yellow 


560 


Light blue 


480 


Deep straw 


570 


Blue 


490 


Yellow brown 


600 


Dark blue 


500 


Brown yellow 


625 


Blue tinged with green 



TEMPERING AND TOUGHENING 193 

Limitation of Color Method. — The previous statement regarding 
the relation of tempering colors to temperature is true in its entirety 
only under certain definite conditions of heating, and which are 
largely dependent upon the time element. So long as the tempera- 
ture of the steel is being progressively raised — that is, so long as the 
temperature of the fire, furnace or tempering plate is greater than the 
temperature of the steel — the temper colors indicate the temperature 
of that part of the steel most affected — the surface. But when the 
steel is being kept at a definite tempering temperature for any length 
of time, the colors do not represent the actual temperature. This 
point is readily illustrated by heating a small piece of hardened steel 
at a constant temperature for a considerable period of time. Thus, 
in one instance, a straw color was produced in about a minute, but 
changed to a brown in about ten minutes, and to a purple in about 
forty minutes; and yet the temperature of the steel was never 
higher than 460° F., representative of the straw color. In other 
words, the time element has developed a new set of conditions wjiich 
may greatly affect the depth of oxidation or color. 

On the other hand, it is a debatable point as to whether or not 
these temper colors represent the actual condition (not the temper- 
ature) of the steel itself. Some tool makers maintain that the 
efficiency of the tool — both in hardness and in other properties — is 
the same whether the color has been obtained by a short heating at a 
high temperature, or a longer heating at a lower temperature. That 
is, the ultimate results are indicated by the temper color, independent 
of the method of obtaining it. Others aver that such is not the case. 

Tempering for Depth. — It is obvious that the temper color is 
at the best but a surface indication. For some tools or articles which 
require a specific superficial hardness only, and in which the condition 
of the center of the tool is of little consequence, it probably does 
not matter a great deal in the ultimate results whether the temper 
color — a straw color for example — has been obtained by a few min- 
utes' heating at 460° F., or by heating for a longer period at say 360° 
F. Contrariwise, if the tool or part is to be subjected to stresses of 
such nature as demand the best that the steel is capable of, the 
greatest degree of uniformity and release of hardening strains is 
requisite. Such may be obtained only by a thorough heating at a 
specified temperature, and which may be entirely independent of the 
color indication. In such cases, to use the above temperatures, the 
thorough heating at 360 ° — it more uniformly affecting the whole 
mass of the steel — might prove immeasurably better than the 



194 STEEL AND ITS HEAT TREATMENT 

incidental surface heating to 460°. And as will be mentioned later, 
a continued heating at 460° would again be an improvement over 
either color method. 

Quenching after Tempering. — The method of tempering by color 
indication inherently requires immersion when the specified color 
is reached to prevent any further rise in temperature, or in the 
blacksmith's phrase, to " set the grain." Although it is possible 
so carefully to heat the steel that the maximum effect is just to 
develop the color desired — and no further, such methods take so 
much time and patience that they are rarely carried out in practice. 
The necessity of such immersion or quenching, even in the hands 
of an experienced hardener, is the source of many troubles. Not only 
does the quenching probably induce further strains into the steel, 
but it is also entirely inconsistent with uniformity of results. If the 
object is of considerable size, or varies greatly in dimension of 
adjoining sections to be similarly tempered, or is of intricate design, 
the difficulty in obtaining the same temper throughout even on the 
surface (to say nothing of the interior of the steel), will be greatly 
magnified. If the proper color is reached on one part before another, 
there will be a corresponding difference in hardness. And thus the 
difficulties multiply ad infinitum. 

Use of Liquid Baths. — Later methods involving the use of liquid 
baths for heating overcome the difficulties in color tempering, 
eliminate — as a general rule — the necessity for quenching, and 
further give complete uniformity of heating throughout the whole 
mass of the steel and the maximum elimination of hardening strains 
as can be obtained at the temperature used. By maintaining the 
bath at the proper temperature there can be no overheating, the 
heat must penetrate all parts of the steel alike, and the " personal 
equation " is as nearly eliminated as is possible. This method has 
the further advantage of cutting down labor costs and increasing 
the output, since a number of pieces may be heated at the same 
time, and while one lot is being tempered another bath may be 
charged or discharged. 

Comparison of Physical Properties Obtained. — An excellent 
example of the efficiency of bath tempering is illustrated in auto- 
mobile gears. On account of the relatively thin section of the teeth 
as compared with the mass of the gear, exact tempering by the ordi- 
nary temper-color practice is rather difficult. The teeth, which 
should be the hardest, take the temper first, and are therefore the 
softest part of the gear as a whole. If the gears were to be tern- 



TEMPERING AND TOUGHENING 195 

pered by revolving on a hot bar much better results would be ob- 
tained than by ordinary tempering, but the time and cost elements 
would prove excessive where hundreds of pieces were to be handled. 
By the use of a suitable liquid tempering bath thorough uniformity 
could be obtained throughout. Where by the color method, the core 
of the gear would have the tendency to be too hard, the teeth per- 
haps too brittle or soft in places, and only the surface of the gear 
as a whole affected by the temper-color representing say 475° F., 
by the more modern method the whole mass of the gear would have 
the physical properties as characterized the drawing temperature 
of the 475° F. 

Exact Temperatures. — Too much attention cannot be given to 
the necessity of obtaining exact temperatures in the tempering opera- 
tion. For the average run of carbon tools the tempering range is 
very narrow, probably within a hundred degrees for the great 
majority. The tempering action takes place extremely rapidly and 
often a difference of 15° or 20° may cause much trouble. Trying 
to temper tools over an open fire may be all right in isolated 
cases, but it spells failure if made a general practice. 

Tempering Methods. — The procedure to be employed in temper- 
ing must necessarily depend upon the nature of the tool or part. 
Methods must be developed to satisfy the individual requirements 
and are too numerous to discuss here. Briefly, however, the more 
common practices may be covered by the tempering plate, the sand 
bath, and such liquid baths as oil, lead and alloys, and molten 
salts. 

Tempering Plate. — The tempering plate generally consists of 
an iron casting planed smooth on top, and heated from beneath by 
suitable means, such as gas, oil, or even a coal or coke fire. The 
steel articles are placed on the plate and moved about until they 
have attained the proper temper color and then quenched. Fig. 155 
shows a characteristic equipment for heating, hardening and temper- 
ing dies; Q represents the discharge end of the heating furnace, 
R the quenching tank, and T the tempering plate, the latter being 
heated by oil burners from beneath. 

Sand Bath^ — In order to effect more uniform tempering of small 
tools, a pan of clean, well-dried sand may be placed on a suitable 
hot-plate, or in a furnace. The sand is held at the desired tem- 
perature, which may be determined by the insertion of a ther- 
mometer or pyrometer couple, and may be protected by covering 
with a suitable hood. The oxide colors on the steel may also be 



196 



STEEL AND ITS HEAT TREATMENT 



used as a measure of the tempering, as there is of course free 
access of air between the particles of the sand. 

Oil Baths. — For much of the ordinary tempering work an oil 
bath will probably prove as satisfactory as any method for temper- 
atures up to about 500° F. or even higher. The chief requisites are 
a tank holding an ample supply of oil, a suitable furnace or method 
of heating by which accurate and constant temperatures may be 
obtained, and a mercury thermometer for determining the tempera- 
ture of the oil. Mineral oil with a flash-point of some 600° F. is 




Fig. 155. — Quenching and Tempering Dies. (Lake, in "Machinery.'") 



generally used for the bath; certain of the animal and vegetable oils 
also are used occasionally. 

Handling the Material. — Oil baths, and similarly the salt 
baths, are provided with a wire basket in which the pieces to 
be tempered are placed and which is then lowered into the oil. 
By this method a number of pieces may be tempered at once, 
besides preventing the steel from coming in contact with the sides 
or bottom of the tank, which is apt to be hotter than the oil- 
It is advisable, whenever possible, to allow the hardened steel to 



TEMPERING AND TOUGHENING 197 

come up gradually to the desired temperature, and not to immerse 
in the oil when the latter is already at the highest heat. Rather 
put the steel in the oil when the latter is about 200° to 300° F. and 
let the two heat up together. The reason for this is that the pre- 
heating — if it may be thus termed — allows the heat to penetrate 
more gradually, softening the outer portion of the steel in such a 
way that the inner and stressed part may be relieved more gradually 
and thus avoiding the danger of fracture. Sudden heating has the 
tendency to set up new stresses which must in turn be overcome. 
The length of time allowed for the tempering to take place will 
depend upon the size and nature of the piece under treatment; 
fifteen minutes or so after the maximum temperature has been 
reached will generally be sufficient for the average run of small 
tools, gears, etc., while larger parts require more time in proportion. 
If large and small parts are tempered at the same time it will do no 
harm to the small pieces if they are not removed until the larger 
pieces are ready, although on general principles long-continued 
heating is never desirable after the steel has responded to the desired 
heating. When the full effect of the tempering has been attained, 
the pieces may then be removed from the oil and allowed to cool off 
in the air, for if the steel has been thoroughly heated at the maximum 
temperature of the tempering operation, no further change will 
take place in the ordinary steels; each phase of the transition is 
represented by a definite temperature for each steel, so that no 
further step in the transition will occur unless the temperature is 
raised — with the possible theoretic exception of very long-continued 
heating. For some large work, such as die blocks, large cutters, 
etc., the steel is allowed to cool off in the oil in order to procure the 
greatest elimination of strains. 

Salt Baths. — If higher drawing temperatures than those possible 
with oil are desired, a bath of salts may be used. A combination of 
two parts of potassium nitrate and three parts of sodium nitrate 
melts at about 450° F. and may be used up to about 1000° F. 
Methods of heating and using are similar to those with oil baths, and 
described under Hardening Baths. The use of nitrate salts instead 
of the chloride salts is necessary on account of the lower tempera- 
ture desired. 

Lead Baths; Alloys. — Lead, having a melting-point of about 
610° to 630° F., may also be used for tempering where temperatures 
higher than its melting-point are required. The disadvantages are 
similar to those noted under its use for heating for hardening. The 



19S 



STEEL AND ITS HEAT TREATMENT 



melting-point may be lowered by alloying the lead with tin, and 
temperatures suitable for ordinary tempering may be obtained 
approximately as follows: l 



Lead 


Tin 


Approx. Melt- 


Lead 


Tin 


Approx. Melt- 


Parts. 


Parts. 


ing Temp. ° F. 


Parts. 


Parts. 


ing Temp. ° F. 


14 


8 


420 


28 


8 


490 


15 


8 


430 


38 


8 


510 


16 


8 


440 


60 


8 


530 


17 


8 


450 


96 


8 


550 


18,5 


8 


460 


200 


8 


560 


20 


8 


470 


Melted lead 




610 to 630 


24 


8 


480 









The use of these various alloys of predetermined melting-points 
for tempering is similar to that previously explained when selecting 
a combination of salts with certain melting-point in the hardening 
operation. 

TOUGHENING 

Sorbite . — As the reheating or drawing temperature is increased 
still further beyond the tempering range we find that another stage 
in the austenitic transition commences — the change of troostite 
into sorbite. Like the change from martensite to troostite, the 
formation of sorbite does not take place spontaneously throughout 
the whole steel, but increases gradually and progressively. Most 
writers believe that sorbite is essentially an uncoagulated conglomer- 
ate of irresoluble pear lite with ferrite in hypo-eutectoid (less than 
about 0.85 per cent, carbon), and cementite in hyper-eutectoid steels 
respectively, but that it often contains some incompletely trans- 
formed matter. Its components at all times tend to coagulate 
into pearlite. On higher heating, sorbite (Fig. 156) changes into 
sorbitic pearlite (Fig. 157), then slowly into granular pearlite (Fig. 
158), and probably indirectly into lamellar pearlite (Fig. 159). Sor- 
bite differs from troostite in that it is softer for a given carbon con- 
tent, and in usually being associated with pearlite instead of martens- 
ite, and from pearlite in being irresoluble into separate particles of 
ferrite and cementite. 

Importance of Sorbite. — The main importance of sorbite is 
due to its physical properties. Although slightly less ductile 
than pearlitic steel for a given carbon content, its tenacity and 

1 Table by O. M. Becker, using melting-point of lead as 610° F. 



TEMPERING AND TOUGHENING 



199 



elastic limit are so high that a higher combination of these three 
properties can be had in sorbitic than in perlitic steels. Steels 




Fig. 156.— Sorbite. X100. (Bullens.) 




Fig. 157.— Sorbitic Pearlite. X100. (Bullens.) 



which are so treated as to contain sorbite are often called " tough- 
ened " steel. 



200 



STEEL AND ITS HEAT TREATMENT 



Toughening Range. — The transition of troostite — the chief 
characteristic of tempered steel, into sorbite— characteristic of 




Fig. 158.— Granular Pearlite. X100. (Bullens.) 




Fig. 159.— Lamellar Pearlite. X650. (Bullens.) 



toughened steel, is gradual, and progresses with the increase and 
duration of the reheating. At some point, depending upon the 



TEMPERING AND TOUGHENING 201 

composition of the steel and the degree to which the steel has been 
affected by the hardening process, sorbite is formed. If we accept 
sorbite as the characteristic constituent of toughened steel (and 
which it undoubtedly is), we may then consider as the lower limit 
of the toughening range that temperature which will produce sor- 
bite. In fully hardened steel of the medium forging and higher 
carbon analyses, characteristic sorbite begins to form at about 
750° F. At about 1250° to 1300° F. the sorbite coagulates into 
pearlite, which is distinctive of annealed steel. With these facts 
in view we may then consider, in a general way, that the toughening 
range lays approximately between 750° and 1250° F. It must be 
remembered, nevertheless, that these temperatures are in no sense 
definite, but are taken arbitrarily as representative of a class of heat- 
treatment work: differences in chemical composition, the degree of 
hardening, the size of work, etc., all play their part. 

Influence of Toughening. — When a piece of hardened steel is 
reheated for toughening, each specific temperature has a certain 
definite influence upon the steel. The results of this toughening 
process are interpreted by the ability of the steel to do certain work, 
to withstand the application of stated loads, or as measured by 
standard methods of testing. On account of the almost universal 
use of the last named for purposes of comparison, we will deal briefly 
with (1) the static strength, as measured by the tensile strength 
and elastic limit, (2) the ductility, as measured by the percentage 
elongation and reduction of area, and (3) the dynamic strength, as 
measured by the alternating impact test. 

Effect of Increased Temperature. — Each increase in the toughen- 
ing temperature lowers the tensile strength and elastic limit, but with 
a corresponding increase in the ductility and dynamic endurance. 
With the majority of ordinary carbon, nickel, chrome and vanadium 
steels the ratio of the elastic limit to the tensile strength remains 
very nearly constant throughout the sorbitic range (which we 
assumed to be approximately from 750° to about 1250° F.). Be- 
yond these temperatures, and coincident with the formation of pearl- 
ite, the values for the elastic limit and tensile strength — of each par- 
ticular steel — begin noticeably to diverge until they reach their 
smallest ratio in fully annealed steel. Up to near the end of the 
sorbitic range the graphs obtained by plotting the elastic limit and 
tensile strength against the drawing temperatures are, for general 
purposes, straight lines, but beyond this range curve towards the 
horizontal, as is represented in Fig. 160. 



202 



STEEL AND ITS HEAT TREATMENT 




TEMPERING AND TOUGHENING 203 

Effect on Ductility. — These changes are accompanied by reverse 
changes in the ductility, as measured by the reduction of area and 
elongation. Research work would tend to show that these two factors 
differ from each other in that the reduction of area generally reaches 
a maximum at about the end of the sorbitic range and then decreases, 
while the elongation does not attain its maximum until the steel is 
fully annealed or in the pearlitic condition. Be this as it may, through 
the sorbitic stage at least, each increment of decrease in tensile 
strength and elastic limit is associated with, and counterbalanced 
by, an increase in the reduction of area and elongation. This com- 
bination of static strength and ductility is further almost directly 
proportional to the toughening temperature. 

Effect of Saturation. — The reduction of area is undoubtedly a 
measure of the heat saturation, and while it is possible to obtain the 
tensile strength, elastic limit and elongation indicative of a definite 
toughening temperature by means of a minimum heating at that 
temperature, the reduction of area is wonderfully increased by pro- 
longed saturation. This, and the practical benefits obtained from 
it, has been brought out very clearly by the experience of rifle barrel 
manufacturers: barrels (lyj in. diameter) which had been heated 
for one-half to one hour at the drawing temperature would pass 
the physical test, including a reasonable reduction of area, but when 
it came to the machining, and particularly the drilling and rifling, 
great difficulty was experienced and many rejections resulted. Bar- 
rels which had been " soaked " at the same temperature for two to 
three hours showed approximately the same tensile strength, elastic 
limit and elongation as with the shorter saturation, but further gave a 
large increase in the reduction of area, and also perfect drilling qual- 
ities. Prolonged saturation was the secret of the heat treatment 
process in this case, and many other examples might be given to 
prove the tremendous advantage of thorough saturation. 

Impact Strength. — The effect of toughening upon other properties 
and especially in relation to the impact strength, is shown in Fig. 
160, rearranged from the work of Grard. The steels, approximating 
0.15, 0.40 and 0.50 per cent, carbon, were hardened and then reheated 
to temperatures varying from no tempering up to 2200° F. The 
impact strength curves present some extremely interesting facts. 
We find that the greatest resistance to shock to be obtained from 
a toughening, after hardening, at a temperature about 100° F. 
below the upper critical range (Ac3); annealing at a temperature 
superior to the Ac3 range gives a lower impact strength. Further, 



204 STEEL AND ITS HEAT TREATMENT 

as the temperature is raised more and more and overheating results, 
there is a marked diminution in the impact strength. Increase in 
the carbon content, assuming the same heat treatment, diminishes 
the impact strength. Tempering (reheating up to say 600° F.) 
has little or no effect upon the impact strength. As a general prop- 
osition we may sum up by stating that it is preferable, in order 
to obtain the greatest impact strength, to keep the carbon content 
low and to have a high drawing temperature. 

Capacity of the Steel. — Thus it will be seen that by changing the 
drawing temperature the grouping of these factors may be varied 
through a considerable range and limited only by what we may call, 
for want of a better phrase, the " capacity of the steel." This 
quantity is defined largely by the chemical composition, the method 
of manufacture, the size of the piece to be treated, and by other 
subordinate factors. With these qualifying conditions in mind, we 
may further define the capacity of the steel as the limiting ratio of 
strength to ductility. Each steel, as qualified above, has certain 
definite limits within which the physical properties may be varied. 
At one end of the see-saw, as in hardened steel, there is a maximum 
tensile strength with minimum ductility; and at the other extreme, 
as in fully annealed or sorbitic-pearlitic steel, there will be a mini- 
mum tensile strength with maximum ductility. Following out the 
simile of the see-saw, we may place tenacity on one end and ductility 
on the other; when one is up, the other must be down; both cannot 
be up nor both down at one and the same time; raise one and the 
other must fall. The heat-treatment man now stands on the middle 
of the board and by means of his reheating temperature can adjust 
the opposing factors to that position which he desires; but he cannot 
change the maximum and minimum of either, because they are fixed 
by the limitations previously mentioned at the beginning of the 
paragraph, and over these he has no control as far as the individual 
steel is concerned. 

Duplication of Results. — Happily for the heat-treatment man, 
each grouping is distinctive of a definite toughening temperature, 
other conditions being the same. When he has once determined the 
relation existing between static strength, ductility, and temperature, 
for a given size piece of work made from a steel of specific analysis, 
he knows that he can duplicate approximately his results under like 
conditions at any time. Not that he can absolutely and ultra- 
scientifically obtain results within a few pounds elastic limit or hun- 
dredths of a per cent, elongation — for such are neither necessary 
nor expected — but that he can reasonably expect to get a com- 



TEMPERING AND TOUGHENING 205 

mercially acceptable duplication. It is with this thought in mind 
that the subsequent chapters have been developed, giving under 
each steel many of the results and details which have been obtained 
in practice and experiment, and which should prove advantageous 
to the average heat-treatment man as a time-saver. 

Slow Cooling and Stresses and Strains. — It is one of the incon- 
trovertible facts of heat-treatment work that slow cooling predicates 
the release of internal stresses and strains. Not only is this true 
of the full-annealing process — as indicative of slow cooling from a 
temperature above that of the critical range, but also of the toughen- 
ing operation. In fact, the very nature of the usefulness of tough- 
ened steel depends upon the absence of a state of strain just as much 
as upon specific static or dynamic properties. Strange as it may 
seem, some of the failures in locomotive forgings may be traced 
back to the lack of slow cooling after toughening; and this trouble 
is coming to be recognized in many specifications by the require- 
ment of cooling in the furnace after toughening. Just as the dangers 
in hardening increase with the rapidity of cooling, carbon content 
and size of section, so are they likewise magnified in cooling after 
toughening — although on a smaller scale. If these factors become 
noticeably important, cooling in air from the toughening tempera- 
ture may set up such a new series of cooling strains that many of 
the real advantages of toughening may be invalidated. 

Use of Furnace Cooling. — The greater part of hardened and 
toughened work, such as automobile and other small forgings, may 
not require furnace cooling, besides being economically impracticable. 
But even with these it is desirable that the pieces should be piled 
together after removal from the furnace so that the cooling will be 
retarded. For forgings of section greater than 3 or 4 ins., such as 
heavy machine parts, ordnance, etc., cooling in the furnace is 
always desirable. It may be said that such slow cooling never did 
any harm, and it may do a world of good in relieving strains. 

Effect of Furnace Cooling on Physical Properties. — Investigation 
would tend to show that slow cooling in the furnace has no noticeable 
tendency to " soften " further the usual straight carbon or alloy 
steels to which the toughening process generally is applied. That 
is, for similar pieces of the same steel treated alike, equivalent 
physical test results would be obtained in the forging which had 
been furnace cooled as in the one which had been allowed to cool in 
the air — the tests being taken from the same relative position. In 
making this statement there is, however, one other necessary quali- 



206 STEEL AND ITS HEAT TREATMENT 

fication : it is assumed that the whole mass of the steel has been thor- 
oughly heated at the toughening temperature. Otherwise the effect 
of the toughening would not be so great in the air-cooled piece as in 
the slowly cooled piece, for the latter would have greater opportunity 
to be affected by the heat of the furnace during the furnace cooling. 
During the toughening range the effect of the heat upon the transition, 
except for very large pieces, practically ceases as soon as the source of 
heat is removed — as by air cooling. 

High vs. Low Toughening Temperatures. — On the hypothesis 
that either of two specified analyses would prove equally satisfactory, 
under suitable treatment, for the same piece of work, but that on 
account of the difference in chemical composition one steel would 
require toughening at say 1200° F. and the other at say 800° or 
900° F., the selection of the higher drawing-point steel should be 
made. Such conditions often arise in heat-treatment plants handling 
a variety of commercial work and it may be well to sum up briefly 
the reasons for the above conclusion. 

The more stable the state of equilibrium which exists between 
the transition constituents the more lasting and effectual will be the 
treatment. Further, the smaller the amount of internal strains 
which may remain in the steel from the previous hardening operation 
the better. Both of these conditions more nearly are brought about 
by the higher drawing temperature. 

As there is also a decided tendency for the dynamic strength 
to reach a maximum at about 1200° to 1300° F. it is probable 
that the higher drawing temperature steel will have a greater 
dynamic strength than the other steel, provided that there is not 
too much difference between the chemical compositions of the two 
steels. 

From the furnace man's point of view the temperatures around 
1200°, being of characteristic visible reds, are recognizable decidedly 
more easily than those temperatures around 800° to 1000°, since 
with these lower temperatures there is very little visible heat color. 
The higher drawing temperatures, therefore, aid in the efficiency of 
judging the heating operation and lead to greater uniformity of 
control and of results. 

Quenching Medium vs. Toughening Temperature. — -There is 
another phase of the high or low toughening temperature proposition 
which cannot be solved by any general rule, but only after due 
consideration of all the circumstances involved; this relates to the 
condition of affairs when there is no opportunity for the choice of 



TEMPERING AND TOUGHENING 207 

steel, but depends more upon the selection of the quenching medium 
in relation to the toughening temperature. As we have noted, 
water quenching gives a harder steel than oil quenching. It natu- 
rally follows that, in order to obtain approximately the same physi- 
cal results, the oil-quenched piece must be drawn at a lower temper- 
ature than the water-quenched piece. The arguments regarding 
water vs. oil quenching, and low vs. high drawing temperatures have 
been discussed previously. If the solution were to be developed 
entirely along these lines it is probable that in the majority of cases 
the oil quenching (giving less hardening strains) and lower drawing 
temperature would be employed. In other words, the difficulties 
to be encountered with water quenching — the hardening operation 
being the more drastic of the two — would more than outweigh the 
the disadvantages of the lower toughening temperature. This is a 
question in which the personal element and experience of the heat- 
treatment man would be paramount. 

Influence of the Carbon Content. — In respect to the selection of 
the steel in relation to the treatment there remains the consideration 
of the influence of the carbon content. Carbon not only intensifies 
the effect of the rapid cooling (hardening), but it also directly 
augments the brittleness of the steel. Or, to put it in other words, 
the greater the carbon content the greater the hardening strains, 
and the lower the ductility which can be obtained with a stated 
tensile strength. It therefore usually is desirable to provide a steel 
with as low a carbon content as will give the desired results. 

Toughening vs. Annealing. — It is only within comparatively 
recent years that the toughening process with its attendant sorbitic 
structure has been used and understood. Previously, annealing 
was generally the cure-all for brittleness and a strained condition 
of the steel. Pearlite — produced by annealing — on account of its 
entangled structure, gives a large measure of ductility; but also 
gives a minimum tenacity. The appearance of sorbite, however, is 
even more entangled than pearlite; sorbite is far superior to pearlite 
in tensile strength and especially in elastic limit. Thus by obtaining 
a sorbitic steel by suitable treatment, almost as much ductility, 
greater working strength, greater dynamic strength, and — by being 
able to use a lower carbon steel — less brittleness may be obtained 
than in a pearlitic or annealed steel. 

Standardization of Results. — With the same degree of hardening, 
and if the reheating has been uniform and thorough at a given tem- 
perature, the physical results will be comparatively the same for 



208 



STEEL AND ITS HEAT TREATMENT 



material of equivalent section and the same composition. That 
is, the product will be standard for standardized treatment. Fur- 
ther, in order to get standard results with steel purchased under the 
same general specifications (i.e., each chemical constituent within 
certain limits) , the toughening temperatures may be varied according 
to the chemical composition. To illustrate: the following heats of 
steel of varying chemical composition and made by several steel 
companies were manufactured into a certain product which, when 
heat treated, required an elastic limit of 85,000 to 95,000 lbs. per 
square inch, and an elongation of not less than 16 per cent, in 
2 ins. In spite of the varying carbon, manganese, chrome and 
nickel contents, the toughening temperatures (maintained within 
5° F. under or over) were so adjusted as to give the desired results. 
Thousands of pieces, some weighing as much as 200 lbs., all ful- 
filled, by actual test, the standard physical specifications. 



Carbon. 


Manga- 
nese. 


Phosphorus. 


Sulphur. 


Chrome. 


Nickel. 


Toughen- 
ing Temp. 
Deg. Fahr. 


0.16 


0.43 


0.015 


0.017 


0.62 


1.82 


1050 


.185 


.44 


.010 


.015 


.57 


1.56 


975 


.20 


.43 


.009 


.016 


.64 


1.74 


1025 


.20 


.46 


.011 


.017 


.40 


1.56 


950 


.21 


.48 


.015 


.015 


.60 


1.77 


1050 


.21 


.50 


.017 


.014 


.67 


1.84 


1075 


.23 


.50 


.016 


.018 


.65 


1.73 


1075 


.245 


.53 


.015 


.020 


.62 


1.79 


1120 


.25 


.50 


.011 


.019 


.64 


1.40 


1140 


.26 


.43 


.010 


.018 


.60 


1.65 


1100 


27 


.49 


.015 


.021 


.63 


1.79 


1150 


.28 


.51 


.008 


.011 


.41 


1.57 


1120 



Quench-Toughening. — A process which has been used consider- 
ably for the treatment of large forgings of uniform section, such as 
heavy axles, is that of heating as usual for hardening and then 
quenching in oil for a specified number of seconds, followed by air 
cooling. The oil quenching affects the steel to a certain depth, but 
still leaves a considerable amount of heat in the' forging when 
removed from the bath. As the forging cools in the air this heat 
from within will toughen or " soften " the steel affected by the 
quenching. In order to obtain equivalent results under varying 
conditions the number of seconds required for immersion in the oil 
of a piece of given size must be determined by experiment and strictly 



TEMPERING AND TOUGHENING 209 

adhered to. Forgings treated by this process are characterized 
by a soft or annealed core, with a progressively toughened outer 
part. 

Physical Results. — In subsequent chapters will be given results 
obtained in actual practice by the use of various toughening tem- 
peratures for different grades of steel. 



CHAPTER X 1 
CASE CARBURIZING 

Object of Case Hardening. — The object of case hardening or 
partial cementation is the production of a hard wearing surface (the 
" case ") on low carbon steel, and at the same time the retention or 
increase of the toughness of the " core " of the metal. The process 
may be roughly divided into two distinct periods. First, the car- 
burization or impregnation of the surface by which the carbon con- 
tent is sufficiently raised — dependent upon the demands of the work 
—so as to give a steel capable of taking on very great surface hardness. 
Second, suitable heat treatment which shall develop the properties 
of both case and core. The complete operation should not only 
result in the obtaining of a very hard case, but also and simultaneously 
in the realization of special mechanical properties in the core — more 
especially that of non-brittleness. Briefly, the aim is to have a 
piece of steel which shall possess a minimum fragility and a maximum 
surface hardness. 

Requirements for Case Carburizing. — In order to obtain a case 
rich in carbon, the metal is heated in the presence of a body which is 
capable of delivering this carbon, by more or less complex reactions, 
which is then dissolved by the steel. Aside from the use of gases 
in the newer processes involving such factors as pressure, quantity, 
etc., there are four main factors which must be considered in the 
carburizing operation: 

1. The solvent: that is, the steel; 

2. The product to be dissolved, or more exactly, the compound 

capable of delivering the carbon, i.e., the cement; 

3. The temperature; 

4. The time of contact between the steel and the carburizing 

agent. 

2 Cuts by Giolitti from " The Cementation of Iron and Steel," by courtesy of 
McGraw-Hill Book Co.; references made in this chapter to investigations by 
Giolitti are also from the above. 

210 



CASE CARBURIZING 211 



THE STEEL 

The Steel. — The character of the initial steel used for case car- 
burizing depends largely upon the fact that one of the main desires 
is to eliminate brittleness in the core. We have seen that any 
increase of carbon, other conditions being equal, will increase the 
brittleness, particularly when the carbon content is raised to over 
about 0.25 per cent. Further, as practically all commercial car- 
burizing processes involving case hardening are followed by one or 
more hardening operations, it follows that the use of a steel with a 
higher carbon content will also increase the brittleness through 
quenching. For these reasons it is therefore necessary to keep 
the carbon content of the steel to be carburized quite low, prefer- 
ably under 0.25 per cent, for straight carbon steels. In fact, the best 
French practice is to demand a carbon content of not over 0.12 per 
cent., further qualified by the specifications that the core after 
quenching shall give a tensile strength of about 54,000 lbs. per square 
inch and not to exceed 85,000 lbs., together with an elongation of 
30 per cent, in 100 mm. (3.94 ins.). 

However, one of the important and often unsatisfactory results 
of using an extra-soft steel is the difficulty encountered in machining 
(before carburizing) . If the carbon is extremely low the steel is 
very apt to tear, and thus increasing the amount of grinding after 
hardening — in order to obtain a perfectly smooth surface. For 
this reason, the general American practice is to adopt a carbon 
content about midway between the extreme upper and lower limits 
and specify a steel with about 0.16 to 0.22 per cent, carbon. The 
higher carbons also give increased stiffness to the core which, in 
some cases, is necessary. 

It is generally recognized that the carbon content, at least 
up to some 0.50 per cent., has no influence upon the velocity of 
penetration of the carburization, i.e., the depth of carburization 
which will be obtained for a given length of exposure. 

On the other hand, the initial carbon content of the steel will 
have a decided influence upon the maximum carbon content which 
will be obtained in the case; the higher the initial carbon, the higher 
the maximum carbon concentration in the case. 

Manganese. — It is considered the best practice, in general, to 
require a low manganese content with about 0.30 to 0.35 per cent, 
as the maximum. It should be remembered that the case which 
will be formed during the carburization will be characteristic of 



212 STEEL AND ITS HEAT TREATMENT 

a high-duty tool steel and will have the properties of such. Thus 
manganese will increase the hardness of the case (and also of the 
core) and will make the steel as a whole more sensitive to rapid cool- 
ing. In spite of this, it is often customary, especially in British 
practice, to use a manganese content of about 0.70 per cent. — and 
in some cases even up to 0.90 per cent.- — in order to obtain greater 
stiffness in the core. Manganese at such percentages also increases 
the brittleness produced by long heating during carburization, and 
diminishes the efficacy of the regenerative quenching. These last 
named points are also true when the silicon is much over 0.30 per 
cent. 

Other Impurities. — It is self-evident that the content of phos- 
phorus and sulphur in the initial steel should be just as low as is 
possible. Slag, blow-holes, segregation, and all other impurities 
and imperfections should be entirely absent from steels for case 
hardening. 

THE CEMENT 

Direct Action of Carbon. — Carburization by its very nature 
requires the presence of free carbon in some form or other, either as 
a solid body, or as some gas which will produce free carbon by its 
decomposition. The mere presence of free carbon in contact with 
iron, however, will not satisfy the conditions necessary for commercial 
carburization. Although it has been shown scientifically that car- 
bon alone, without the intervention of any gas, will carburize iron 
if it is kept in contact with it for a sufficiently long time and at a 
sufficiently high temperature, this direct action, as far as industrial 
results are concerned, is negligible. That is, the ordinary forms 
of solid carbon, such as wood charcoal, sugar charcoal, etc., exercise 
directly on iron but a very slight carburizing action in the absence of 
gases. 

Action of Gases. — It will be noted that emphasis has been laid 
upon the " direct action " in the " absence of gases." This at once 
leads to the question as to what is meant by the action of gases, and 
which, in turn, involves the mechanism of cementation itself. It is 
a well-known fact that when steel is heated, the " pores of the steel 
are opened " — to use the vernacular expression — it becomes easily 
permeable to gases, and the surrounding gases diffuse into the steel. 
This is true whether the steel is heated in the ordinary atmosphere, 
when the gases consist of nitrogen and oxygen, or whether it is heated 
in some specially prepared atmosphere, such as carbon monoxide, 



CASE CARBURIZING 213 

illuminating gas, etc. The main fact to be realized is that the gases 
do penetrate into the steel, although the effect of the gases will 
depend upon the composition of the gas, besides such other factors 
as pressure, temperature, and so forth. Thus, recognizing that the 
direct action of carbon — that is, the carburizing results obtained by 
mere contact of carbon with iron — is commercially negligible in the 
absence of gases, it is evident that carburization must be intimately 
related to the presence of gases. In other words, the gases (or, more 
exactly, certain gases) must in themselves act as the carrier or 
vehicle for the carbon. That this carrier action, or transporting of 
the carbon, has not been definitely recognized or determined until 
recently has been due to the fact that practically all of the solid 
cements generate the necessary gases through their own decomposi- 
tion and interaction with the occluded air. Further, the intense 
and critical study of this action has been developed only by the 
research work in connection with the newer processes of case 
carburizing by means of gases alone. 

Action of Oxygen. — As a typic 1 example of this diffusion and its 
effect we may consider any ordinary carburization process in which 
wood charcoal is used as the base cement. When the carburizing 
material and articles are packed in the carburization boxes there is 
necessarily a considerable quantity of air also occluded with the 
particles of the cement. Under the influence of heat the oxygen of 
the occluded air will react with the carbon or charcoal to form car- 
bon monoxide gas, which has the symbol CO. Then, as the tem- 
perature of the box and contents increases to the temperature of the 
carburization proper, these gases of carbon monoxide permeate or 
diffuse through the surface and outer section of the steel. At the 
same time, by catalytic action, the carbon monoxide gas decom- 
poses when it comes in contact with the steel and sets free a part 
of the carbon it contains. This decomposition may be represented 
by the reversible reaction 

2CO t± C0 2 + C 

carbon monoxide^carbon dioxide (gas) + carbon (solid). 

Thus, as the gas diffuses into the mass of the steel it continues to 
decompose, setting free new quantities of carbon within the steel. 
This carbon, at the proper temperatures of carburization, passes 
directly into solution in the steel and forms a true steel proper. The 
reaction above, being reversible — as might be shown — will continue 
indefinitely under suitable conditions, the charcoal regenerating the 



214 STEEL AND ITS HEAT TREATMENT 

supply of carbon monoxide. Further, while it is a well-known fact 
that carbon monoxide, acting alone on iron, will deposit free carbon 
on the surface of the iron, this action takes place only at temperatures 
lower than those ordinarily used for commercial cementation. In 
other words, the carburizing action of charcoal as used in practice 
is not due to the direct action of the carbon, but is due (under the 
conditions named, which of course may be modified by the presence 
of other gases or components of the cement) entirely to the specific 
action of carbon monoxide as a gas. 

Nitrogen.— The action of the oxygen of the occluded air being 
accounted for, the accompanying constituent nitrogen must be con- 
sidered. Although it has been shown that during carburization the 
nitrogen may and will diffuse in small amounts into the steel, it is 
now certain that the presence of pure nitrogen does not increase, 
except to a minimum extent, the carburizing action of free carbon. 
In fact, instead of nitrogen being requisite — as many still believe — 
it may even exert a pernicious effect. LeChatelier has suggested that 
the increase in brittleness sometimes observed in those parts of the 
steel subjected to cementation, but which the carburization has not 
even reached, may be due to this nitrogen. It might be added that 
this deleterious nitrogenizing theory is further supported by experi- 
ments along other lines — particularly in the apparent cleansing effect 
for nitrogen of the titanium additions to steel during manufacture. 
Another general effect of nitrogen gas is to reduce the cementing 
action of the carbon monoxide mentioned by its diluting the car- 
burizing gas. For practical purposes of carburization, however, 
the action of nitrogen in the presence of free carbon is too slight to 
influence commercially the results obtained with a given cement, 
unless actually added (in gaseous cementation) as a diluent. 

Carbonates. — The ash of the carbonaceous matter may also 
contain carbonates of the alkali or alkaline-earth metals. Or these 
carbonates, such as barium carbonate, may be added directly to the 
cement. In the light of the most reliable and recent researches it 
would appear, contrary to previously accepted theories, that the 
activity of these carbonates is not due to the formation of volatile 
cyanides by the action of the nitrogen of the occluded air, but exclu- 
sively to the formation of carbon monoxide produced by the action 
of the hot carbon on the carbon dioxide produced through the dis- 
sociation of the carbonates. Thus the effect of such carbonates 
is similar to that produced by carbon monoxide under similar con- 
ditions. 



CASE CARBURIZING 215 

Cyanides. — The most maligned constituents of cements are the 
cyanogen group. In the past it has been thought that the deriva- 
tives of this group played the chief part in carburization processes. 
This, however, has been strongly disproved by Giolitti, who ad- 
mirably explains the matter as follows: That cyanogen and the 
more or less volatile cyanides can cement iron intensely is beyond 
doubt. Moreover, it is well known that fused potassium and potas- 
sium ferrocyanide are used in the pure state to obtain thin and 
strongly carburized zones (as in superficial carburization or cyanide 
hardening). In industrial practice the cyanides do not exist already 
formed, but may be formed in very small quantity by the action of 
the nitrogen of the air (occluded in the cement) on the carbon used 
and on the small quantities of alkali constituting a part of the ashes 
of this carbon. Although the formation of small quantities of alkali 
cements cannot therefore be wholly avoided in industrial car- 
burization with carbon as a base, the part which is played by these 
traces of volatile cyanides is certainly negligible in comparison with 
that of the carbon monoxide formed by the action of the air on the 
carbon used as cement. — He then submits conclusive proofs to 
substantiate these statements. 

Carbon Monoxide Gas. — Carburization carried out by the use of 
carbon monoxide gas alone will give a mild or gradual carburization 
in which the maximum carbon content is comparatively low — 
not usually reaching the eutectoid ratio even at the periphery — and 
which diminishes progressively and in a uniform and slow manner 
passing from the surface of the case toward the interior of the car- 
burized piece. Carburized zones of this type correspond always 
and only to carburization carried on with pure carbon monoxide, 
a concentration-depth diagram of which is shown in Fig. 161. On 
account of its definite chemical composition and simplicity of action, 
the general behavior of carbon monoxide is known within almost 
exact limits. The carburizing action is easily regulated, and the 
case may be obtained with certainty with any kind of steel in com- 
mercial use. 

When working under suitable conditions, carbon monoxide — 
either alone or with a mixture in which the carbon monoxide can exer- 
cise its maximum carburizing action — will give the greatest velocity 
of carburization, i.e., the depth reached in a given time by the car- 
burized zone. This depth is also a direct function of the time or 
length of exposure. 

All other conditions being equal, the higher the temperature of 



216 



STEEL AND ITS HEAT TREATMENT 



carburization using carbon monoxide, the smaller will be the maxi- 
mum carbon content of the case. Similarly, the lower the pressure 
of the carbon monoxide, the smaller the maximum carbon content; 
and the greater the quantity of pure carbon monoxide gas coming- 
in contact with a unit of surface, the greater the carbon concentra- 
tion. 

Under suitable conditions, carbon monoxide gas will deposit 
no carbon on the surface of the steel being carburized, so that there 
is little difficulty in keeping the surface bright. Further, the use of 
carbon monoxide reduces to a minimum the deformations and varia- 
tions in volume due to the carburizing processes. Carbon monoxide 
also lends itself in obtaining a good protection of the parts of the steel 
which it is not desired to carburize. 



1.0 
0.8 
0.6 
0.4 

0.2 























































































0.5 1 1.5 2 2.5 3 .MM, 
Fig. 161. — Carburization at 2010° F. for Ten Hours with Carbon Monoxide 

(Giolitti.) 

Hydrocarbons.- Most of the forms of solid carbon used in prac- 
tical carburization are not pure, but may contain organic residues not 
wholly decomposed, or considerable proportions of ash rich in cer- 
tain carbonates. Thus charred bone, charred leather and similar 
organic products often used, will, under the influence of heat, evolve 
hydrocarbons. These hydrocarbons, by more or less complex 
reactions, deposit the excess of finely divided carbon which they con- 
tain on the surface of the metal; and this, in turn, being in perfect 
contact with the metal, at high temperatures may cause a direct 
carburization by contact. But further and vastly more important 
than this direct action of the carbon deposit on the surface of the 
metal, is the carburization by means of the specific action of the gas 
itself, although of course depending more specifically upon the exact 
conditions of carburization. In a manner somewhat analogous to 
that of the decomposition of the carbon monoxide within the steel, 



CASE CARBURIZING 



217 



yielding carbon directly to the steel, the hydrocarbon gases will also 
diffuse into the steel and there yield carbon. Hydrocarbons there- 
fore also act as carriers for the carbon and effect a carburization due 
to the specific action of the gas. 

Carburization with pure hydrocarbon gases give cases of a type 
corresponding to Figs. 162 and 163, and to Fig. 164. These are 
characterized on slow cooling by (1) a layer or zone of hyper-eutectoid 
steel consisting of free cementite and pearlite ; (2) by a layer of eutec- 
toid steel, generally quite thin; and (3) by an internal layer of 
hypo-eutectoid steel. The main points to be noticed are, that 



1.4 
1.2 
1.0 

0.8 
0.6 
1 






















































































0.2 































0.5 1 1.5 2 2.5 3 MM. 
Fig. 162.— Carburization at 1830° F. for Five Hours with Ethylene. (Giolitti.) 

the case contains a structure with greater than 0.9 per cent, carbon, 
and more emphatically, that the concentration of the carbon often 
diminishes in a markedly non-uniform manner or discontinuity. 
Zones of this type are always found in carburizations carried out 
with hydrocarbons; they also are typical of carburizations obtained 
with many of the solid carburizing compounds used in commercial 
work in which the action of the hydrocarbons greatly predominates, 
or in the presence of cyanides (superficial cementation). 

Of the specific action of the gaseous hydrocarbons, we may make 
the following remarks. The depth or velocity of penetration in- 
creases, similarly to carbon monoxide, with the time of exposure. 
In the case of carburization with ethylene and methane, the cemented 
zones obtained in a definite time, although likewise increasing 
markedly in thickness with rise in temperature, other things remaining 
constant, maintain about the same concentration and the same dis- 



218 STEEL AND ITS HEAT TREATMENT 

tribution of the carbon in the three zones — thus differing widely 
from carbon monoxide. In contrast with the use of these pure 
gases, the use of hydrocarbons in practice presents a different aspect, 
especially when compared with the use of carbon monoxide in prac- 
tice. Contrary to the simplicity of the reactions which always 
characterize the cementation by carbon monoxide, the complexity 
of the reactions with hydrocarbons increases enormously in industrial 
work. The gas in such instances does not consist of a single, chem- 
ically definite hydrocarbon, but of a mixture of various hydrocarbons. 
If we work at a comparatively low temperature, such as at, or slightly 






X-:- 



Fig. 163. — Carburization with Hydro-carbons. X25. (Bullens.) 

under, the upper critical range, the process is slow and non-uniform. 
At the high temperatures generally used, cemented zones of exces- 
sively high carbon are always produced. The same complexity of 
reactions make it difficult, in practice, to work with a cement having 
hydrocarbons as a base, either as a mixture of solids in the carburizing 
box, or as gases in the newer processes, in such a way as to obtain 
well-defined results. Thus the use of such hydrocarbons is not 
advantageous where a certain value of maximum concentration, 
combined with a definite distribution of that carbon, is necessary 
in carburized steels in which a considerable depth is desired. 

Enfoliation. — All those who have had much to do with case 
hardening and its products are familiar with the flaking, chipping, or 
even peeling off of parts of the case from the remainder of the steel. 



CASE CARBURIZING 



219 



These fractures are entirely different from those occurring in homo- 
geneous high-carbon hardened steels. While in the latter the frac- 
tures always have a characteristic conchoidal form, in case-hardened 
steels the chipping or enfoliation always takes place along a line 
corresponding to the separation of two zones exhibiting markedly 
different structure or " grain." A microscopic and chemical investi- 
gation brings out the fact that this line or plane of weakness charac- 
terizes the separation of the hyper-eutectoid zone from that of the 
hypo-eutectoid zone, or at a carbon content corresponding to that of 
about 0.90 per cent. Further, this plane of weakness corresponds 
to a discontinuity in the concentration or distribution of the carbon 



1.2 
U) 

0.8 

0.0 
0.4 
0.2 



















































































































0.5 1 1,5 2 2.5' 3 MM. 
Fig. 164.— Carburization at 1920° F. for Four Hours with Ethylene. (Giolitti.) 



which is characteristic of carburized zones of the hydrocarbon type 
previously described. 

It is now evident that, in order to eliminate the possibility and 
dangers of this enfoliation, we must 

(1) obtain a gradual and progressive change in the distribution 
of the carbon so that it will vary from the minimum of the core to 
the maximum at the surface of the case, and in no place exhibit the 
phenomenon of discontinuity; 

(2) eliminate the possibility of discontinuity at the eutectoid by 
keeping the maximum concentration of the carbon at or below 0.90 
per cent, carbon (thus eliminating the hyper-eutectoid zone) • 

(8) and in any case, modify by suitable heat treatments the 
structure obtained by carburization, 



220 STEEL AND ITS HEAT TREATMENT 

Maximum Carbon Concentration. — Now while we have under 
(2) advised the eutectoid carbon ratio as a means of preventing 
enfoliation, it must not be at once concluded that enfoliation is the 
direct sequence of increasing the carbon concentration maximum 
to over 0.9 per cent. Such is not the case if the proper heat treatment 
methods are employed. Unfortunately, however, the majority of 
commercial plants employing case hardening do not either under- 
stand, or are unable to put into practice, the methods which are 
necessary when the carbon content of the case runs beyond 0.9 per 
cent, carbon. That such high carbon contents are undeniably 
advantageous in many instances where it has been generally thought 
that their use was impossible will also be shown, as will the so-called 
" secret " processes of treating the steel. But for plants which are 
unable to employ the necessary metallurgical skill and appliances, 
it will be far better to adopt such case-hardening processes as will 
turn out a good product having a maximum carbon concentration in 
the case of about 0.9 per cent. The further advantages of this will 
be brought out under the discussion of heat-treatment methods in 
the next chapter. 

Intermediary Type of Carburized Zone. — Recognizing under these 
conditions the validity of not exceeding the eutectoid limit, and the 
obvious advantages of preventing discontinuity between the core and 
the surface of the case regardless of the maximum carbon content, 
it is evident that we must obtain a cemented zone intermediary 
between those of the two general types, previously described. In 
other words, the type of case must have the principal character- 
istics of the carbon monoxide type, but which are modified — by 
increasing the carbon content — by cements typical, of the hydro- 
carbons, or other suitable procedure. 

Carbon Monoxide Plus Hydrocarbons.— From the results of 
experiments carried out with carbon monoxide plus specific amounts 
of hydrocarbons, Giolitti shows that the additive effect of the latter, 
as compared with those carried out with pure carbon monoxide, may 
be summed up as follows: " The addition of small quantities of 
volatile hydrocarbons to carbon monoxide merely raises the con- 
centration of the carbon in the external layers of the cemented zones 
above the value which would result from the use of pure carbon 
monoxide under identical experimental conditions. This increase 
is greater the larger the proportion of the hydrocarbon contained in 
the gaseous mixture, as long as this proportion does not reach 
a value such that the velocity with which the free carbon is formed 



CASE CARBURIZING 



221 



by the decomposition of the hydrocarbon does not surpass the veloc- 
ity with which this carbon passes through the stage of carbon mon- 
oxide into solution in the iron. From this limit the excess of carbon 
which is liberated begins to deposit on the steel and the concentration 
of the carbon in the external layers of the cemented zone reaches the 
maximum value corresponding to that which is obtained by cement- 
ing with solid cements, or with cements which behave as such, and 
from this point on, the concentration and the distribution of the 
carbon in the cemented zones no longer vary markedly, even if the 
proportion of the hydrocarbon increases greatly." 

" From what precedes it is evidently possible to obtain, by means 
of mixtures of carbon monoxide and vapors of volatile hydrocarbons, 
cemented zones in which the maximum concentration of the carbon 



0.8 
0.6 

0.4 

0.2 






.2 .4 .6 .8 1 1.2 1.4 1.6 1.8 2 MM. 

Fig. 165. — -Cemented Zone, Intermediate Type, Carburized with Carbon Monox- 
ide Plus 3.1 per cent. Ethylene. (Giolitti.) 



in the external layers has a definite value, lying between a minimum 
corresponding to that which would be obtained by working under 
the given conditions with pure carbon monoxide, and a maximum 
which would be obtained by working with vapors of the hydrocarbon 
alone. This is achieved simply by using gaseous mixtures containing 
a proper proportion of hydrocarbon varying with the conditions 
under which cementation is to be effected, such as temperature, 
pressure, relation between the velocity of the gaseous current and 
the surface of the steel to be cemented, etc." 

An example of this is shown by the concentration-depth diagram 
in Fig. 165, the results of which were obtained experimentally by 
cementing 0.26 per cent, carbon steel cylinders for four hours at a 
temperature of 1830° F. in a mixture of carbon monoxide with 3.1 
per cent, of ethylene. It will be noted that the carbon decreases 
progressively and in a slow and uniform manner, but that the addi- 



222 STEEL AND ITS HEAT TREATMENT 

tion of the hydrocarbon has raised the maximum carbon content up 
to nearly the eutectoid ratio. Thus, in this case, there has been pro- 
duced a carburized zone of an intermediary type which fulfils the 
requirements stated for the avoidance of enfoliation. 

Following along these lines of using a gaseous mixture consisting 
of certain proportions of carbon monoxide gas and the volatile 
hydrocarbons, several industrial methods have been worked out, and 
which have given excellent satisfaction. The application of the 
same theory is also applicable to the commercial solid cements in 
which the necessary gases are evolved during the heating operation, 
but on account of the greater lack of control the variations to be 
obtained are necessarily of considerable extent. 

Carbon Plus Carbon Monoxide. — As we have stated, the carburiz- 
ing action of solid carbon in the absence of all gases is commercially 
negligible. But by introducing oxygen which will form the gaseous 
vehicle (carbon monoxide), or by adding carbon monoxide directly, 
the presence of solid carbon greatly intensifies the carburization. 
Thus, similarly to definite mixtures of carbon monoxide plus hydro- 
carbons, the desirable form of the intermediary type of carburized 
zone may be obtained by carbon monoxide in the presence of solid 
cements. 

By varying the various factors of temperature, time of exposure, 
pressure of gas, etc., the use of a mixed cement may be varied within 
wide limits, and with the production of a hyper-eutectic zone if so 
desired. Tins latter comes into great practical use when it is desired 
to produce zones of considerable width. Co-ordinated with this is 
the use of carbon monoxide as an " equalizer," that is, by first carry- 
ing out the carburization process in the usual way (with mixed 
cements), the maximum concentration of the carbon may be made 
quite high; this is then followed by the use of carbon monoxide alone 
(without the presence of granular carbon). By these means the 
concentration of the carbon may be lowered — by the distributive 
action of the carbon monoxide, to such maximum concentration as 
may be desired. This is shown graphically in Fig. 166, by Giolitti. 
The steel used was of the composition: 

Per cent. 

Carbon 0.12 

Manganese . 47 

Phosphorus . 03 

Sulphur 0. 02 

Silicon 0.06 



CASE CARBURIZING 



223 



Curve a shows the concentration depth after carburization for ten 
hours at 2010° F. with mixed cement. Curve b represents the 
results after heating the preceding for five hours at the same tem- 
perature in " isolated " carbon monoxide. Curve c gives the results 
after another five hours heating at the same temperature in "isolated" 
carbon monoxide. Thus we see that the curves have undergone 
a gradual change in form and position due to the action of carbon 
monoxide alone. Such methods as these will permit of the elimina- 
tion of the dangerous hyper-eutectoid zone, and at the same time give 




Fig. 



12 3456789 MM. 

166. — Distributive Action of Carbon Monoxide. (Giolitti.) 



all the benefits to be obtained from a carburized zone of the inter- 
mediate type previously described. 



TEMPERATURE AND TIME FACTORS 

Solution of the Carbon. — We have seen how certain gases, by 
diffusing into the steel, precipitate free carbon within the steel. 
Now this carbon, under suitable conditions, may be dissolved at once 
by the iron, forming a true steel. It is evident that the solubility of 
this carbon (or carbide) must depend upon the allotropic condition of 
the iron, which, in turn, will depend upon the tempeiature. As we 
have explained in previous chapters, iron may be held in the alpha, 
beta or gamma state. Thus, if a piece of normal 0.2 per cent, carbon 



224 STEEL AND ITS HEAT TREATMENT 

steel is heated, none of the cementite which is mechanically mixed 
with ferrite (iron) to make up the mechanical mixture pearlite is 
affected until the lower critical temperature of about 1350° F. is 
reached. At this temperature the iron of the pearlite, previously 
in the alpha condition, changes into gamma iron and dissolves the 
cementite, the two forming a solid solution or austenite. In other 
words, it is necessary for the iron to be in a higher allotropic condition 
than that of the alpha stage in order to dissolve carbon (or carbide) . 
As the temperature of the steel is progressively raised, more and more 
of the excess iron is dissolved by the austenite, until at the Ac3 range 
the whole mass of the steel consists of austenite and has all the 
iron in the gamma state. 

Thus we see that while it is possible for carburization to take 
place at temperatures varying between the Acl and Ac3 ranges, 
the carburization must necessarily be not only slow, but also irreg- 
ular and non-uniform. In other words, the minimum temperature 
which should be used' in industrial carburization should not be 
lower than the upper critical range of the initial steel to be 
carburized. 

Depth of Penetration. — All commercial carburizing processes 
must provide a depth of case which will satisfy the requirements of 
the specific use to which the steel will be put in practice. Thus 
many parts will only require a depth of case of say ^j or -^ of an 
inch, or if grinding is necessary, this may be increased to y§- of an 
inch; other parts, such as armor plate, may require a considerable 
thickness. With a definite depth of case in view, economic consider- 
ations require that the velocity of penetration shall be definitely 
known in relation to the factors of time and temperature, the nature 
of the carburizing agent, and — in the case of gases — such other factors 
as pressure, quantity of gas, etc. . 

The penetration of carbon (differentiating this depth of penetra- 
tion from the distribution of the carbon) increases with the temper- 
ature and with the time of exposure, but not always in direct pro- 
portion to these two factors. Given a definite temperature and 
carburizing compound, it may be said in general that 'the carburiza- 
tion commences and continues at a comparatively high rate of speed 
until the outer layers are saturated with carbon — dependent, of 
course, upon the nature of the cement; there is then a drop in the 
rate of carburization, vaiying according to the temperature, and this 
in turn is followed by a velocity of penetration which seems to be 
more nearly proportional to the length of exposure. 



CASE CARUBRIZING 



225 



These facts are shown graphically in Figs. 167 and 168, the former 
illustrating this velocity of penetration particularly for short expos- 
ures, while the latter emphasizes the effect of long continued heating. 
In each case the experiments were carried out under conditions usu- 
ally adopted in industrial establishments. The bars of soft steel 
were allowed to soak at the stated temperatures for definite lengths 
of time and the penetration was then measured. The results from 
which these graphs were plotted were reported, in the first instance, 





















.12 

.10 

ro .08 

01 

js 
o 

a 

M 

.06 




























^> 
































J65CP. 
















1550 




.04 
.02 





































3 4 5 

Hours 



Fig. 167. — Velocity of Penetration, Short Exposures. 



by a company using a cement of the barium carbonate-carbon- 
aceous type, and in the second case by Giolitti, using a common 
commercial cement consisting of ground-wood charcoal treated 
with 5 per cent, potassium ferrocyanide and mixed with an equal 
weight of barium carbonate. 

In the case of the first compound it is interesting to note that 
there is a decided decrease in the relative rate of penetration when 
the depth of case reaches approximately 0.05 in. This cycle 
appears to take place in the same order at all temperatures used, 
with the difference that the relative speeds of penetration are higher 



226 



STEEL AND ITS HEAT TREATMENT 



at higher temperatures, although not proportionally so. Thus we 
may coin a phrase and call this depth of 0.05 hi. the ." critical 
penetration" of this individual compound. Considering the depth 
of penetration only, and disregarding other economic and technical 
factors which may enter into consideration, it is evident that this 
particular compound may be used to good advantage when a case 
corresponding to about 0.05 in. is desired. To this depth the 
steel will be carburized at a maximum speed or velocity, and there- 



m 

o 


























o 

e 

M 

.14 
.12 
.10 
.08 

.00 
.04 

.02 
















V 


* 






a 






































































^ 


f>° 



































































































12 24 36 48 60 72 84 96 108 120 Hours 
Fig. 168. — Velocity of Penetration, Long Exposures. (Giolitti.) 



fore at the minimum furnace or heating cost. With this particular 
compound simply as an illustration, any other commercial carburizing 
compound might be studied from the practical side for the determina- 
tion of this critical depth of penetration, and use made of the results 
to good advantage in reducing operation costs. 

The influence of temperatures of carburization under the upper 
critical range upon the depth of penetration and the maximum car- 
bon content is also brought out by a comparison of Figs. 169 and 170. 
These photomicrographs represent the effect of cementation upon a 
0.11 per cent, carbon steel carburized for one hour in a mixture of 



CASE CARBURIZING 



227 



charcoal and barium carbonate, but at different temperatures. 
Fig. 169 shows the results of a temperature under the A3 range; 
Fig. 170 the cementation at a temperature considerably over the A3 
range. The practical value is self-evident. 

Liquation. — Sudden variations in the concentration of the carbon 
in the cemented zone may be manifested when intense carburiza- 
tion is effected at high temperatures, and the carburized pieces are 
allowed to cool slowly through a more or less wide interval of tem- 







t-- — r: 









Fig. 169. — Carburization of 0.11 per 
cent. Carbon Steel at a Temper- 
ature under the Upper Critical 
Range with BaC0 3 and Char- 
coal, for One Hour. (Nolly 
and Veyret.) 




Fig, 170. — Carburization of 0.11 per 
cent. Carbon Steel at a Temper- 
ature Considerably Over the 
Upper Critical Range with 
BaCo 3 and Charcoal, for One 
Hour. (Nolly and Veyret.) 



perature before being quenched. This variation consists of a true 
liquation of the cementite (and of the ferrite) during their segrega- 
tion from the solid solution. Take, for example, the diagrams in 
Figs. 171 and 172, representing the results obtained by carburizing a 
0.26 per cent, carbon steel for four hours at 1830° F. in ethylene, with 
the difference, however, that the carburized steel represented by 
Fig. 171 was cooled — during 32 minutes — to a temperature of 1380° F. 
and then quenched, while that of Fig. 172 was quenched immediately 
following carburization from that temperature (1830°). 

Comparing the two diagrams we see that, while the concentration 
of the carbon in Fig. 172 decreases continuously and uniformly as we 



228 



STEEL AND ITS HEAT TREATMENT 



proceed from the surface towards the core, that in Fig. 171 shows a 
marked increase, followed by a very rapid decrease, before it exhibits 



JK3. 


























1.6 


























1.4 
1.2 
1.0 










































































0.8 


























0.6 
0.4 
0.2 











































































.2 .4 ."6 .8 1 1.2 1.4 1.6 1.8 2 2.2 MM. 

Fig. 171. — Liquation of Cementite through Slow Cooling. (Giolitti.) 

that gradual decrease which characterized the first case. As the 
carburization proper was identical in both cases, it is evident that 



go. 

1,6 
L4 
1.2 
1.0 
0.8 
0.6 
0.4 
02 




























^ 








































































































































































i 






.2 A .6 .8 1 1.2 1.4 1.6 1.8 MM. 
Fig. 172. — Prevention of Liquation by Quenching. (Giolitti.) 

the discontinuity in the carbon distribution must be due to the dif- 
ference in the rate of cooling following the carburization proper. 



CASE CARBURIZING 



229 



Thus we see that the liquation, or accumulation, of the cementite 
in the external layers will tend to emphasize the line of demarkation 
between the hyper- and hypo-eutectoid zones. And this, in turn, will 
magnify the dangers of enfoliation. Fig. 173 is a photomicrograph 
from a piece of carburized steel which failed in service; the cause 
of enfoliation in this case is undoubtedly due to this phenomenon 
of liquation. 

This is another reason why those processes of carburization should 
be used which will avoid the formation of the hyper-eutectoid zone, 











1* 










Fig. 173. — Failure Due to Liquation. (Giolitti.) 



and thus eliminate the possibility of enfoliation through differences 
in carbon distribution caused by accumulation of the free cementite. 
However, if cements which give carburized zones of the hydro- 
carbon type are used, it is apparent that this accumulation may be 
avoided by quenching from the temperature of carburization. 
Although this is possible in the newer processes of using either a 
gaseous mixture or a mixed cement, it is manifestly impossible under 
the older methods of using solid cements. We will shortly describe 
methods of heat treatment by which the effect of the cementite 
accumulation may be corrected in the majority of instances, as well 



230 STEEL AND ITS HEAT TREATMENT 

as giving reasons why quenching from the temperature of carburiza- 
tion — when it is 1750° F. or more — is technically bad. 

Oscillating Temperatures. — There is still another effect of tem- 
perature which should be mentioned in this connection on account 
of its action in many industrial carburizing processes. This is the 
effect of non-uniform or oscillating temperatures during carburiza- 
tion, so often met with in practice. Without going into the theo- 
retical explanations involved, it has been shown by Giolitti and 
Scavia that, where under normal conditions the formation of free 
cementite cannot take place, by carrying out the carburization under 
identical conditions but with variable temperatures oscillating within 
definite intervals, the occurrence of free cementite will result. The 
industrial importance of this is evident. In the first place it explains 
the abnormal increase in free cementite which is sometimes met with 
in practice in which the normal nature of the carburization should 
produce cemented zones of the intermediate type and the absence of 
free cementite, besides, demonstrating the necessity of maintaining 
uniform temperatures throughout the whole carburization within 
a very close range. And in the second place it furnishes a means of 
carrying out the heating during carburization in such a way as 
purposely to cause, with certainty, the formation of free cementite 
when it is desired to obtain cemented zones capable of taking an 
exceedingly high degree of hardness by quenching without its being 
necessary that their brittleness be reduced to a minimum. 

Temperature Factors. — Although much has been said and 
written concerning the effect of temperature upon carburization, the 
majority have confused the velocity or depth of the carbon concen- 
tration with the maximum concentration and distribution of the car- 
bon. While it has been shown that the depth of penetration is, in 
general, a direct function of the temperature with all commercial 
carburizing mixtures, the same is not entirely true of the maximum 
concentration of the carbon. In fact, through the study of pure 
gases such as we have indicated, it has been shown that in the case 
of carbon monoxide alone the maximum concentration of the carbon 
actually decreases, other things being equal, with increase in the 
temperature of carburization. 

In the cases of the newer processes involving the uses of gases or 
a mixed cement, the effect of such temperature on the maximum 
concentration and distribution of the carbon may be practically 
varied at will by a change in the other factors previously mentioned. 
In other words, on account of the almost absolute control with which 



CASE CARBURIZING 231 

such processes of carburization may be regulated, the actual effect of 
the temperature is minimized and does not play the important part 
which is manifested in the older processes involving the use of solids 
alone. 

Influence of Temperature on Different Cements. — The com- 
plexity and lack of control of the reactions involved in the use of 
solid cements make the factor of temperature extremely important. 
Thus some cements, such as charcoal plus barium carbonate, which 
at the lower temperatures give " gradual " or " mild " cases, at the 
higher temperatures may act as " sudden " or " quick " cements. 
A consideration of the common solid cements (of which we are now 
speaking) as used in general commercial work would tend towards 
the conclusion that their action, on the whole, is more gradual at 
the lower temperatures than at the higher temperatures. Further, 
it might even be said that, other things being equal, the higher the 
temperature of carburization the higher will be the maximum carbon 
concentration in the case. Although these statements may not hold 
good for all instances in which carbon is used as the base, their prac- 
tical working out is generally evidenced in the general average of 
thin cemented zones found in commercial case hardening. 

Low Temperature Carburization. — Now as the majority of case- 
hardened products require the elimination of brittleness, both in 
case and core, and as the use of " gradual " cements at the lower 
temperatures of carburization will advance that condition of affairs 
(i.e., the formation of cemented zones of the intermediate type, 
showing the absence of the hyper-eutectoid-zone, and a gradual 
distribution of the carbon concentration from the external surface 
of the case to the core) the use of the lower temperatures of carbur- 
ization is coming more and more into vogue. That is, the tendency 
is to use moderate heats and maintain them for a length of time 
sufficient to obtain a reasonable depth of case. These heats may be 
said, in a general way, to correspond with temperatures of about 
100° F. over the upper critical range of the steel to be carburized. 
Although the depth of case is largely dependent upon the temper- 
ature, as well as upon the time of carburization, under the above con- 
ditions it should be considered poor practice to raise the temperature 
to the high limit simply for the purpose of reducing the time element; 
the repair items on the furnace will increase, the fuel cost will be 
greater, and — above all- — the maximum carbon concentration and 
the relation of the various zones to each other so changed that 
the whole character of the finished product may be altered, 



232 



STEEL AND ITS HEAT TREATMENT 



Relation of Temperature to Grain-Size. — Another important 
feature in determining whether or not to use a high temperature 
for case carburizing is the relation of such temperature to the grain 
size. As has been explained in previous chapters, upon passing the 
Acl range on heating the grain size begins to coarsen, and most 
noticeably so after passing the upper critical range or the low carbur- 
izing temperatures. Thus from about 1550° F. and so on up to 
1850° or 1900° F. (which are about the maximum temperatures 
used) the grain increases in size most markedly. This increase 
in grain size has a direct bearing upon the impact strength of the 
steel, as is shown by the following results (by Guillet) obtained by 
annealing case-hardening steel bars at the temperatures given for 
eight hours, and which under the conditions of a normal annealing 
gave an impact test of 28 kilogram-meters. 



Temperature, 
° F. 


Impact Test, 
Kilogram-metres. 


1470 


26 


1560 


28 


1650 


15 


1740 


12 


1830 


4 


1920 


3 


2010 


4 



Thus it will be seen that any heating of long duration at temperatures 
above the upper critical range (the low case carburizing temperature) 
greatly lowers the resistance of the steel to impact or shock. When 
high temperature carburizing is necessary, however, suitable treat- 
ment after carburizing may be used to " regenerate " the core. 
Even then, however, the effect of long-continued heating at high 
temperatures is often manifest. 

High-temperature Carburization. — Although the advisability 
of using the lower temperatures for case carburizing has been 
emphasized, it must not be thought that the higher temperatures 
of 1800° F., or even higher, are never to be used. Contrarily, the 
latter are often mandatory in certain classes of work where speed 
of penetration is the first requisite, or where low cost of production 
is necessitated and the absence of brittleness is not a prime factor. 
If overheating and burning are suitably guarded against, and the 
methods of packing are such as will keep warping and distortion at a 
minimum, and the carburizing process is followed by a technically 



CASE CARBURIZING 233 

adjusted series of heat treatment operations, most excellent results 
can be obtained. In fact, it is only within comparatively recent 
years that the use of the lower temperatures has been practiced 
to any great extent; a decade or so ago if 1550° F. or thereabouts 
had been suggested as giving greater efficiency, the proposal would 
have been laughed at by the majority of " practical " hardeners. 

COMMEECIAL DATA 

Simple Solid Cements. — In the foregoing sections we have 
attempted to give in brief the underlying principles which govern all 
processes of partial carburization. By the use of these principles 
we have further shown that, in carburizations with gaseous or certain 
mixed cements, it is possible to so select the conditions of carburiza- 
tion as to obtain with certainty cemented zones of predetermined 
and definite form. Similarly, the same principles are applicable 
to carburization with solid cements, although (as we have shown) 
it is not possible to even approximate the same accuracy on account 
of the lack of control of such cementations. Nevertheless, a study of 
the preceding pages should convince one of the importance of using 
carburizing compounds, the composition and manner of acting of 
which are definitely known. With such principles and ideas in mind, 
it should be comparatively easy for each one to prepare for himself those 
cements which are much more simple, effective, and of less cost than 
many of those purchased from dealers at high prices, under fancy names, 
and of unknown composition. With these thoughts in mind, we will 
briefly discuss some of the more simple compounds used in commercial 
work. 

Wood Charcoal. — Finely divided carbon is the simplest of the 
solid cements, the purest form in commercial practice being wood 
charcoal. As we have seen, the carburizing activity of powdered 
wood charcoal is dependent upon the formation and action of carbon 
monoxide, and which is further diluted by the nitrogen of the occluded 
air. It is evident that the use of this charcoal in the ordinary- 
short carburizations for the production of cases of ^j to -^ in. in 
thickness will have the tendency to give cemented zones of low 
and irregular carbon content. Its use for the deeper carburizations, 
however, may be distinctly advantageous, as it opposes the formation 
of zones too high in carbon. Figs. 174 and 175 show the effect of 
temperature upon carburizations with wood charcoal, the first figure 
representing carburizing at 1560° F., and the second photomicro- 
graph at 1925° F. 



234 



STEEL AND ITS HEAT TREATMENT 



Animal Charcoal.- — Thus, for thin cases, wood charcoal is gen- 
erally mixed with certain proportions of the less pure charcoals, 
such as those produced by the carbonization or charring of leather, 
bones, hoofs, horns, hair and other animal refuse, etc. In these 
last cements we therefore have the action of pure carbon or char- 
coal greatly intensified through the generation of volatile hydro- 
carbons. We will later mention the influence of the phosphorus 
and sulphur which these cements may contain. By mixing wood 














<*■ ••■:^'*^ -: -':■"• \ ' U^fr* %&£ -mi "Xx- *■'■'& f- -if «*?| 



Fig. 174. — Carburization with Char- 
coal for One Hour at 1560° F., 
0.11 per cent. Carbon Steel 
(Nolly and Veyret.) 



Fig. 175. — Carburization with Char- 
coal for One Hour at 1925° F., 
0.11 per cent. Carbon Steel. 
(Nolly and Veyret.) 



charcoal with definite proportions of these animal charcoals, the 
carburizing action may be roughly adjusted between the minimum 
value to be obtained with wood charcoal and the maximum value 
of the animal charcoals. Of the more common " mild " cements 
thus obtained we may mention the following: 

, Parts 

a. Powdered oak charcoal 5 

Powdered leather charcoal 2 

Lampblack 3 

6. Wood charcoal 7 

Animal charcoal 3 



CASE CARBURIZING 235 

Parte 

c. Powdered beech charcoal 3 

Powdered horn charcoal 2 

Powdered animal charcoal 2 

Common Salt. — Common salt (sodium chloride) is used in many- 
works in addition to charcoal, it seeming to give better results than 
wood charcoal alone. Exactly what is its specific action is not 
thoroughly understood. Thus we have the mixture : 

Parts 

Wood charcoal 7 to 9 

Common salt 3 to 1 

Barium Carbonate. — -One of the best solid cements for general use 
is that consisting of: 

Parts 

Barium carbonate 40 

Powdered wood charcoal 60 

Its action is well known and is as we have previously described. 
For cases of small depths it gives carburized zones markedly more 
homogeneous than those furnished by other solid cements. Giolitti 
sums up its advantages as follows: " In general, the maximum con- 
centration of the carbon in the cemented zones obtained with carbon 
and barium carbonate at temperatures between 1650° and 2010° F. 
varies from a minimum of about 0.7 per cent., for the very thin zones 
obtained near 1650°, to a maximum of about 1.3 per cent, for the 
zones thicker than 1 mm. (0.04 in.) obtained near 2010° F. 

" Another advantage of this cement lies in its property of being 
' regenerated ' easily and spontaneously when it is left exposed in a 
thin layer to the air, after having been used in the usual manner. 
This process of regeneration is due to the fact that the barium oxide 
formed during cementation by the dissociation of the barium car- 
bonate, absorbs carbon dioxide from the air, again forming barium 
carbonate. After a certain number of alternating cementations and 
regenerations it is necessary to add some wood charcoal to the cement 
to replace that burned during the cementation and during the dis- 
charging of the boxes. 

" The preparation of this cement consists simply in finely grind- 
ing and intimately mixing the wood charcoal and barium carbonate. 
If the natural barium carbonate (witherite) is used, it is necessary 
to powder it carefully before adding it to the carbon; the finely 



236 



STEEL AND ITS HEAT TREATMENT 



divided precipitated barium carbonate, on the contrary, can be 
mixed directly with the granulated carbon and the one operation of 
grinding the carbon can be used for preparing the mixture." 

It is, of course, not always necessary to use the above mixture 
ratio of 40-60, although this combination has been shown to give 
about as good results as may be obtained. The conditions of heat- 
ing, temperature, size of the pieces, type of carburization box and 
method of packing, etc., will alter each individual carburization, 
and experiments should be made to determine as exactly as possible 
the proper combination of the different factors of carburization which 



3.5 
3.0 










3 Cf ^ 


0^^ 












& 
■p^ 










as 
O 
o 

o 

CS 

•2 1.5 

oS 

u 

o 

§1.0 

Ph 

0.5 




•c 


V 

4, 














V 


<^ 


vC»» 


.^ — - 














C^-^"^ 











































11 



2 4 6 8 10- 12 

Duration of the Heating (Hours) 

Fig. 176. — Carburization with Common Carburizing Compounds 



16 



(Scott.) 



enter into consideration. One of the governing factors which is often 
overlooked is the action of the charcoal, dependent upon its composi- 
tion. Thus much of the ordinary commercial charcoal still contains 
considerable volatile or organic matter (hydrocarbons) which may 
distinctly alter the effect of the carburizing. In order to reduce 
the intensifying action of such constituents, and to reduce the forma- 
tion of the hyper-eutectoid zone, it is always advisable first to 
calcine the charcoal before using. 

The general relation existing between the depth of penetration 
due to charcoal, charred leather, and the usual 40-60 barium car- 
bonate-charcoal mixture is graphically shown in Fig. 176, obtained by 
Scott in the carburization of soft steel bars at 1650° F. 



CASE CARBURIZING 237 

Gradual Cements.— The cements which we have just enumerated 
are generally classed as " gradual," for reasons previously given. 
Yet on the other hand, these same cements, under different con- 
ditions of carrying out the carburization, act as " sudden " or 
" quick " cements. Thus the barium carbonate mixture when used 
at low temperatures, or for the carburization of pieces of large 
dimension which heat up slowly, may furnish cemented zones in 
which the maximum carbon concentration may not be over the 
eutectoid ratio (0.9 per cent.). The same mixture, on the con- 
trary, may become a sudden cement at the very high temperatures 
and in carburizing objects of small dimensions. 

Other Solid Cements. — In addition to the use of charcoal plus 
the animal charcoals, barium carbonate and common salt, the other 
agents which may be added are innumerable. To give a list of them 
would occupy several pages, besides leading to the inevitable con- 
clusion that the efficacy of the majority of them is small, or might 
be even detrimental. For it should be stated with emphasis that 
the more simply and more chemically definite a cement can be made, 
the greater will be the industrial advantages. 

Sudden Cements. — The nature of the most important of these 
additions is to make the mixture act quickly, giving rise to a thin 
cemented zone of high carbon in a very short interval. Thus we 
have the use of coke saturated with mineral oil, of the saturation of 
the charcoal in solutions of cyanides or ferrocyanides, and of the 
presence in greater or less quantities of the concentrated salts of 
cyanogen as specific additions. Of those used in practice, the follow- 
ing example is extremely interesting; 

11 lbs. prussiate of potash, 
30 lbs. sal soda, 
20 lbs. coarse salt, 

6 bushels powdered hickory charcoal, 
30 quarts water. 

Grentt recommends the following cements which have given good 
results in practice: 

Parts. 

a. Powdered wood charcoal 1 

Salt . . ... i 

Sawdust 1| 



238 STEEL AND ITS HEAT TREATMENT 

Parts 

b. Coal with 30 per cent, volatile matter 5 

Charred leather 5 

Salt 1 

Sawdust 15 

c. Charred leather 10 

Yellow prussiate 2 

Sawdust 10 

The velocity of carburization increases gradually from the first to 
the third of these cements. The sawdust, by making the mass 
more porous, increases the activity of the gases. 

Size of the Carburizing Box. — The selection or general design of 
the box or container for carburizing is worthy of more attention than 
is frequently given to it. In the attempt to get a uniform case, much 
thought and research has been given to the selection of the steel, 
the carburizing mixture and the degree and duration of heating ; and 
yet in many instances it has all proven unavailing. It must be 
remembered that the inside of a small box takes quite a while to 
come up to the temperature of the furnace; and that if a large box 
is used, the material in the center may, and does, lag behind the 
indicated furnace temperature several hours or its time equivalent — 
several hundred degrees. The greater the size of the box, the larger 
will be this error, and the greater the actual difference in the thick- 
ness of case taken on by steel near the sides of the box as compared 
with that near the center of the box. No manipulation of the furnace 
can change this effect; it can only be remedied by altering the 
dimensions of the box itself. Here, then, lies one explanation of 
many unexplained failures. 

The box should not be larger than is absolutely necessary, even 
where large quantities are to be carburized in it. It should be narrow 
in at least one dimension so that the heat has a chance to penetrate 
quickly at least from two sides and reach all the contents at about 
the same time. Further, the boxes should not be made too deep in 
proportion to their other dimensions, as it makes it 'more difficult 
to pack the parts into them if so made. Whenever possible, the 
design of the box should follow the outline of the piece to be carbur- 
ized, allowing about 1 to 2 ins. all around for clearance and packing, 
so that the surfaces may be uniformly heated and carburized alike. 

Material for Boxes. — Malleable iron probably gives as good 
satisfaction as any of the materials used in making the boxes. Cast- 



CASE CARBURIZING 239 

iron boxes, although of comparatively small initial cost, will not stand 
reheating very many times, and have the further objectionable 
feature of being somewhat porous. Soft-steel plates and wrought 
iron may also be made up into good boxes. 

The thickness of the wall forms an important feature of the box, 
for if it is too thin it easily burns through, and if too thick it offers 
too much resistance to the penetration of the heat to the interior. 
For the ordinary size boxes, wall thicknesses of ^ to \ in. are common 
practice. The boxes should be provided with feet so that the heat 
may circulate all around them. The cover should be as close fitting 
as is practicable, and should also be provided with ribs along the top 
to prevent excessive warping. Ribs along the side also add to the 
service of the box, besides making handling with grappling irons 
more easy. The sides of the boxes should taper slightly towards 
the bottom so that the contents can be the more quickly dumped out. 

Packing. — Carefulness in packing is fundamental to good practice 
and uniformity of results, just as much as carefulness in heating or 
treatment. The method of packing should be such as will insure 
as nearly as possible the even heating and uniform carburizing of all 
pieces in the same box. 

The method of packing must necessarily vary with each type 
of article to be handled. Heavy pieces, or pieces of regular shape, 
do not require the care and patience which should be used with pieces 
of intricate design or with those which on account of their size and 
shape may be readily influenced by high temperatures. The packing 
of such pieces must be individualized. For example, long, slender 
pieces should always be packed vertically, so that the pieces will 
be held in position by the carburizing material and cannot sag under 
the influence of the high temperatures. Again, gears and similar 
pieces may be most suitably packed in tubes, so that the same 
amount of carburizing material and the same degree and length of 
heating may influence all parts of the periphery in equal proportion. 
In carburizing screws and bolts it is well to distribute them in 
the box in two opposite rows, each row having the head of the screw 
towards the side of the box and the stem towards the center. New 
compound may be used at the sides and old compound in the center. 
Owing to the difference in heat and the difference in the carburizing 
power of the compounds, this will cause a much deeper carburization 
of the heads than of the stems — which is exactly what is desired. 
Again, should a narrow or low box not be available in connection 
with small work, carburizing compound which has been used once 



240 STEEL AND ITS HEAT TREATMENT 

before may be put next to the sides of the box while the new com- 
pound is placed in the center. In this way the difference in car- 
burizing which might result from the different temperatures in 
various parts of the box may be offset. 

The first step in the general operation of packing is to cover the 
bottom of the box with the compound to a depth of 1| to 2 ins., 
tamping it solidly into place. The parts to be carburized are 
then placed firmly upon this bed so that the compound and work 
are in close contact with each other. The pieces should in no case 
touch the sides of the box, but should be placed about 1 to 1\ ins. 
away from it. Further, the articles should be separated from each 
other by at least \ to 1 in., dependent upon their size and the depth 
of case desired. If the articles should touch one another, it is evident 
that the carburizing action will have less influence at that particular 
point- — with resulting soft spots. Non-uniformity of case may also 
result if there is not sufficient carburizing material in the box; it is 
better to err by using too much than too little. 

After the first layer of work has been placed in the box, it is 
entirely covered with the carburizing compound. This should 
be packed and tamped down around and over the pieces so as to 
have the particles of cement in close contact with the steel, but yet 
not so tightly as to prevent the free circulation of the carburizing 
gases which are generated during the heating process. When the 
first layer has thus been suitably packed and covered, the same 
procedure is repeated until the box is nearly filled. The point to be 
kept in mind is that each and every piece should be surrounded on 
all sides by a suitable amount of the carburizing compound. 

At least 2 ins. of the compound should form the top blanket over 
the last layer of work. Some shops adopt the following, with the 
aim of further preventing the escape of the gases: about 2 ins. from 
the top of the box sheet-steel strips about y§- in. thick are laid over 
the last layer of the carburizing material and these, in turn, are 
covered with about 1 in. of powdered charcoal. When the box is 
finally packed, the cover is placed on the box and the edges are care- 
fully sealed with fire-clay or asbestos cement. The box is now 
ready for the heating operation. 

The Heating. — The two principal points to be mentioned under 
this heading are: the heating, at least up to 1300° F., should be 
gradual; (2) the heating beyond this temperature should be uniform 
over all parts of the carburizing box. It has been shown by several 
experimenters that the energetic liberation of gases commences 



CASE CARBURIZING 241 

very strongly at temperatures somewhat under 1300° F. for the 
majority of solid cements, and it is advisable to diminish this factor 
as much as possible in order to obtain a more gradual cementation. 
Furthermore, it gives more opportunity for the steel to adjust itself 
to the effect of heating. The second point made is self-evident: 
non-uniformity of heating must necessarily result in non-uniformity 
of product. 

Sulphur Diffusion. — The influence of sulphur contained in the 
cements is an extremely important factor in carburization carried 
on with solid cements. Grayson l has produced uncontrovertible 
evidence that sulphur will diffuse into iron at the temperatures 
ordinarily used for carburization with such substances as charred 
leather (which, under the conditions of his case-hardening experi- 
ments, contained 0.55 per cent, total sulphur), and that this sulphur 
combines with the manganese and iron to form manganese and iron 
sulphides. 

Thus, in Fig. 177, which is a photomicrograph of a piece of 0.17 
per cent, carbon steel carburized for six hours at 1650° to 1750° F. 
with charred leather, it will be noticed that on the edge are present, 
in large quantities, sulphide of manganese, also sulphide of iron with 
ferrite crystals intermingled. That this is sulphide was later proven 
by means of silver prints and by analysis — which showed 2.10 per 
cent, of sulphur increase in the first 0.0025 inch. 

This sulphur diffusion is a very serious matter, because when the 
surface is saturated, as in this figure, it tends to produce a soft skin, 
and even if present in smaller proportions it will weaken the structure 
considerably, thus making it very " chippy," consequently causing 
two effects which must essentially be avoided in any case-hardened 
work. 

In Fig. 178, being a similar steel carburized at 1750° to 1830° F., 
the sulphide is again present, but not in such a large proportion; 
thus the higher temperature has volatilized still more of the sul- 
phur from the carburizing material. Fig. 179 shows the same car- 
burized piece as in Fig. 177, but afterwards reheated and quenched 
in water from 1380° F. In this reheating the sulphide tends to 
" ball " itself up, and, if anything, diffuse further in. 

Thus it may be seen that, for proper carburizing, the solid 
cements should be as free from sulphur as is possible. On the other 
hand, the barium carbonate mixtures generally used do not contain 
sulphur, and this sulphur diffusion cannot take place. 
1 S. A. Grayson, Inst. Journ., No. 1, 1910. 



242 



STEEL AND ITS HEAT TREATMENT 



American Gas Furnace Process. — The apparatus for carburizing 
with gas, as devised by the American Gas Furnace Company, is 



"1 




Fig. 177. — Soft Case Due to Sulphur 
Diffusion. (Grayson.) 







Fig. 178. — Sulphides Diffusing 
Further into Case with Higher 
Temperatures of Carburization. 
(Grayson.) 



iT 




Fig. 179. — Sulphide Globules in Carburized Steel after Hardening. (Grayson.) 



shown in Fig. 180. The carburizing machine consists of a carburizing 
retort enclosed by a cylindrical furnace body in which it rotates, 



CASE CARBURIZING 



243 



together with suitable arrangements for charging and discharging 
the work, burners for securing a proper distribution of the fuel, and 
supply pipes for gas and air. The machine shown has a space 
available for work of 30 ins. in length by 7 ins. in diameter. It is 
suitable for work not over 6 ins. in diameter or 20 ins. in length; 




Fig. 180. — American Gas Furnace Co.'s Carburizing Machine. 

for shafts, tubes, mandrels and bars of nearly equal thickness 
throughout of not over 24 ins. in length or 5 ins. in diameter; or for 
small pieces such as screws, washers, discs, etc., of a charge of about 
100 pounds. The machine uses ordinary illuminating gas for both 
heating and carburizing. 

The vertical section, Fig. 181, through the center lengthwise, 



244 



STEEL AND ITS HEAT TREATMENT 



mr 











: P 



CASE CARBURIZING 



245 



shows the heavy wrought-iron retort A, which is slowly rotated on 
the rollers BB by the gear C, in contact with worm D, propelled by 
a sprocket and chain belt. The reference letters EE show air spaces 




in the retort formed by the two pistons 7, between which the work 
is confined, to the properly heated central section of the retort. 
Letters FF indicate the heating space surrounding the retort, into 



246 STEEL AND ITS HEAT TREATMENT 

which the fuel gas and air are injected under pressure, from two rows 
of burners indicated in the upper half of the casing by the letter G. 
The cover H, closing the retort, is connected with the piston-like 
disc marked /, by the pipe J, which is the vent of the retort. The 
cover H and disc / are withdrawn to charge the retort and replaced 
after the work is inserted. 

Carburizing machines connected with an automatic quenching 
bath are shown in Fig. 182. 

SUPERFICIAL HARDENING 

Superficial hardening differs from case carburizing in that in 
the former method the outer and higher carbon section constitutes 
a " skin " of only a few thousandths of an inch in thickness, while 
in the case-carburizing process the carburized zone forms a case 
of noticeable thickness. Exactly the same principles apply, however, 
in both instances, and which have been previously explained. 

Processes. — The superficial hardening processes may be grouped 
under the headings of " cyanide hardening " and " pack hardening. " 

The cyanide hardening processes are essentially used for the pur- 
pose of obtaining an extreme degree of surface hardness (wear) on 
low-carbon or machinery steel, and in which it is not necessary to 
obtain high resistance to shock, etc. 

On the other hand, pack hardening is essentially a method of 
heating used particularly for fine threaded tools and other tool- 
steel work. The process, when correctly carried out, permits of 
uniform heating with the entire elimination of oxidation by surround- 
ing the steel with a carbonaceous packing. But further, by prolong- 
ing the duration of heating at the hardening temperature, a very 
thin skin of higher carbon content may be formed, so that pack 
hardening may develop, either intentionally or otherwise, into a 
superficial hardening process. 

Cyanide Hardening. — In cyanide hardening the superficial car- 
burizing and hardening may be effected by one of two general 
methods: (1) immersion of the object in a bath of liquid potassium 
cyanide or other mixture with cyanogen as the base, followed by 
quenching; (2) coating or sprinkling the surface of the object with an 
adhesive mixture of finely pulverized carburizing cyanogenous salt 
or " varnish," heating the steel to the proper hardening tempera- 
ture — and thus melting the cyanide — and hardening as usual. 
The first or " immersion " process is by far the most efficient, both 
as to uniformity of the carburized zone and simplicity and uniformity 



CASE CARBURIZING 247 

of operation. Further, this first method has the tendency to reduce 
deformation and oxidation during heating and quenching, since, 
as previously explained, heating in anj^ molten bath has this effect. 

The Immersion Methods — The method of cyanide hardening by 
immersion is quite simple. The salt, usually potassium cyanide 
(KCN), is melted in a suitable pot-furnace, and is maintained at a 
temperature a little over the upper critical range of the steel to be 
carburized and hardened. This temperature, for ordinary machin- 
ery steel, is about 1550° to 1600° F. The steel is then immersed 
in the molten cyanide and kept there until it has been uniformly 
heated; or this heating may be somewhat prolonged in order to 
obtain a greater depth of skin. In general, however, it is not advis- 
able to heat for a length of time much greater than ten or fifteen 
minutes, or at temperatures much over the critical range, since such 
heating will tend to give non-uniform and high-carbon zones which, 
after quenching, are intensely brittle and may chip off in service. 
Quenching is usually done in lime water in order to neutralize the 
cyanide remaining on the steel. Some concerns adopt the method 
of immersing the steel in the cyanide as soon as it has become molten, 
permitting the steel to heat up with the bath, and then quenching 
as soon as the desired temperature of say 1550° to 1575° F. has been 
attained. 

It is absolutely necessary to remember that cyanogen compounds 
are deadly poisonous, and every precaution should be adopted when 
using them. Furnaces should be supplied with hoods which have 
strong draft. Gloves should be used in handling all work, for if 
cyanide gets into a fresh cut or scratch it will prove deadly. In 
some cases, when working at the furnaces, it is even advisable to 
use face masks and to cover up any exposed parts of the body. 

Cyanide Hardening Plant. — A battery of twenty cyanide fur- 
naces is shown in Figs. 183 and 184. 1 In front of the first pair of 
furnaces in Fig. 183 two special machines are shown which suddenly 
cool or quench the work as fast as it can be heated and removed 
from the furnace. They are used for hardening the steel ring discs 
shown at U. These alternate with brass discs in a multiple-disc 
clutch on the engine of an automobile. Each pair of furnaces shown 
in these two ; figures is covered with a hood to convey the poisonous 
fumes to the outer atmosphere through pipes extending through the 
roof. In addition to this, sheet-metal shields are located in front of 
the furnace openings shown at V to carry away from the workmen 
1 E. F. Lake, in " Machinery," Sept., 1914. 



248 



STEEL AND ITS HEAT TREATMENT 



any fumes that might come through these openings. (These shields 
were removed for photographing.) At the end of the cyanide fur- 
naces shown in Fig. 184 is a stationary tank of lime water in which 
some of the work is quenched. On the floor is shown a tray loaded 
with bevel differential gears and having a long rod for a handle. This 
is lowered into the cyanide bath to heat the gears, then lifted out and 




Fig. 183. — Battery of Cyanide Furnaces — Special Quenching Machines for 
Clutch Rings in Foreground. (Lake, in " Machinery.") 



lowered into the quenching tank, and when cool the gears are dumped 
into boxes to take to the tempering furnaces. Other parts that are 
being hardened are shown in the metal boxes beside the tray of gears. 
Other Cyanide Methods. — The second general process of cyanide 
hardening, in its simplest form, is to heat the steel to about 1550° F. 
or so; sprinkle upon it, or plunge it into, potassium cyanide or 
potassium ferrocyanide; again heat to 1550° to 1600° F. until the 
cyanide is melted; and then quench in water. In case the amount 
of cyanide obtained the first time is insufficient, the operation previ- 
ous to quenching may be repeated until a layer of the required thick- 
ness is obtained. It is, of course, necessary to have a clean surface, 



CASE CARBURIZING 



249 



free from scale and oxidation, so that the carburizing reactions may 
take place readily. 

Other more elaborate processes based upon the above are in use, 




and involve the application of special carburizing varnishes. On the 
whole, however, the simpler the process or carburizing compound, 
the more efficacious will it be in actual and everyday practice. 

Pack Hardening. — Pack hardening, as a superficial carburizing 
process, so raises the carbon content in the surface of the steel that 



250 STEEL AND ITS HEAT TREATMENT 

the tools may be hardened in oil — instead of in water — and still 
obtain the requisite degree of either cutting or wearing hardness. 
For certain work requiring almost perfect hardening results this 
method cannot be overestimated. In cases in which the required 
degree of hardness may usually be obtained only by the use of water 
quenching, oil quenching may now be used; and with it will be 
associated the toughness of core inherent with the use of oil as a 
quenching medium. Further, on account of the uniformity in heat- 
ing and the use of oil quenching, the tendency to crack or warp is 
largely eliminated. 

The method is, in fact, a case-carburizing process. The packing 
in boxes is carried out in exactly the same manner as is carburization 
with solid cements, and similar precautions should be used to prevent 
the tools being jarred out of position or touching each other. In 
pack hardening, however, to each tool or piece of steel should be 
attached a wire, so that the tool may be removed promptly to the 
quenching bath when the requisite degree and duration of heat has 
been attained. 

The temperature to be used should be but slightly over the 
critical range of the steel, thus differing from the higher temper- 
atures which are customary in case-carburizing processes. As the 
pack-hardening process is usually used for steels of tool-steel analysis, 
this temperature will be about 1375° to 1400° F. The length of 
time required for the heating will, of course, depend upon the size 
and number of the pieces to be treated in one box, and the depth of 
skin desired; for ordinary small tools this will generally be about 
two hours after the proper temperature has been attained by the 
steel itself. 

Packing material which would be harmful to the steel should not 
be used. Bone, for example, usually contains phosphorus, which is 
apt to make the steel brittle — although burnt bone is not as high in 
this element as is raw bone. Sulphur must also be guarded against. 
If the initial steel does not contain more than 1.20 or 1.25 per cent, 
carbon, charred leather makes a very good packing material. If 
the carbon content exceeds these values, charred hoofs, or a mixture 
of charred hoofs and horns is better than charred leather, since the 
latter will under such conditions have the tendency to give a too 
highly carburized and brittle zone. 

The temperature of the steel in the box may be gauged by means 
of test rods the same size as the tools, or by test wires, or by suitable 
pyrometer equipment. 



CHAPTER XI 
CASE HARDENING: THERMAL TREATMENT 

Heat-treatment Requirements. — It may be said that practically 
all objects which have undergone the case-car burizing processes 
previously described require a subsequent heat treatment of some 
nature. As one of the essential aims of the case-hardening process 
is to produce a hard-wearing surface, and as carburized steels through 
their slow cooling from high temperatures will be more or less lack- 
ing in this necessary hardness, it is evident that a hardening process 
is necessary. In dealing with the subject of case hardening we will 
therefore assume that the carburized steel must undergo some 
hardening process or processes which will bring about this desired 
condition of affairs. 

Secondly, in order that we may at once differentiate the ultimate 
aims of such hardening, and simplify our discussion, we will assume 
that we also desire to obtain a minimum brittleness in both case and 
core. Previous explanations prove that this condition requires that 
the " grain size " be reduced to a minimum, that is, that the steel 
as a whole must be refined. 

To sum up, the specific aims which we have in view require that 
the heat treatment shall combine hardening and grain refinement. 

Comparison with Homogeneous Steels. — The heat treatment of a 
carburized steel differs from that of a homogeneous steel only in the 
fact that, instead of considering the influence of such treatment 
upon one steel, we really have to do with two, or even three main 
classes of steels at once. That is, the carburized steel consists of 
(1) the core, or low-carbon steel; (2) the carburized zones of the case 
with about 0.9 per cent, carbon as the maximum; and, in many 
instances, (3) the carburized zones of the case which, under conditions 
of slow cooling from the temperature of carburization, contain an 
excess of free cementite, i.e., greater than 0.9 per cent, carbon. Our 
heat treatment must therefore be adjusted so as to superimpose 
the effect of one class upon the other. 

Now as the present tendency of case carburizing in industrial 
practice is to preclude the formation of zones containing free cement- 

251 



252 STEEL AND ITS HEAT TREATMENT 

ite (class 3), we will postpone the discussion of the heat treatment 
which involves that class, and thus further simplify matters. We 
now, therefore, have but to consider the related heat treatment of 
steels of very low-carbon content and those containing the eutectoid 
ratio of carbon as a maximum. 

Effect of the Temperature of Carburization. — Into this heat 
treatment there now enters the factor of the temperature of car- 
burization, and its specific influence upon the size of grain in the 
two classes of steels. In the first place, it is axiomatic that the 
effect of any heating at, or slightly above, the Ac3 range for the soft 
steels, and the Acl.2.3 range for the hard steels, is to produce the 
maximum grain refinement (unless such heating is extremely pro- 
longed) . And further, that the effect of any heating at temperatures 
considerably above these temperatures is to produce a size of grain of 
proportionally greater size for the respective steels. (In this chapter 
we are using the phrase " grain size " in its general colloquial mean- 
ing.) Thus, if carburization were carried out at 1600° F. for a case 
carburizing steel of 0.15 per cent, carbon— that is, at a temperature 
but slightly over that of the upper critical range of the initial steel — 
it would produce the minimum grain size in the steel of the core, 
and a certain and proportionally greater grain size in the steel of the 
case. Steels carburized at the lower temperatures we will call 
Group A. Steels carburized at the higher temperatures or about 
1800° F. — that is, considerably over that of the upper critical range — 
we will call Group B, the grain size of both core and case being 
proportionally greater than the minimum. 

Classification. — We now have a further means of classifying our 
heat-treatment processes according to the temperature which was 
used in carburization because of the effect of such temperatures on 
the refinement of the steel. That is, with carburizations of Group 
A, the steel of the core will already be refined, and we need only 
consider the refining of the case; while in Group B the steel of both 
core and case will require refinement. Both groups will, of course, 
require the hardening of the case. 

Treatment of Group A. — Assuming the conditions as in Group A 
(carburization at a temperature slightly over the upper critical range 
of the initial steel), it is evident that the complete heat treatment 
following carburizing will only require the hardening and refining 
of the case, in which, by previous assumption, the maximum car- 
bon content is about 0.9 per cent. A consideration of the principles 
of heat treatment at once shows us that a single quenching at about 



CASE HARDENING: THERMAL TREATMENT 253 

1400° F, that is, slightly over the Al.2.3 range, will bring about the 
fulfillment of both of these conditions. Further, the quenching at 
this temperature, and under existing conditions, will not affect the 
present refinement of the core, nor — if the carbon content is low — 
will it increase the brittleness due to the changing of the pearlite 
of the core into martensite (in fact, it has the opposite effect of in- 
creasing the toughness in the very low carbon steels). Thus, by this 
single quenching, we have completed the requirements originally 
demanded. 

Treatment of Group B. — Turning now to case carburizations at 
the higher temperatures, Group B, it is evident that in addition to 
the refining and hardening of the case we must also refine or regener- 
ate the core. This, we know, may be best accomplished by quench- 
ing the previously cooled steel at a temperature slightly above the 
Ac3 range of the steel of the core. This quenching will put the entire 
steel, both case and core, in the martensitic condition, refine the 
core, but not refine the case. By following this first quenching by 
the quenching at the lower temperature described in the previous 
paragraph we accomplish the following : the hardness and refinement 
of the case reaches a maximum; the refinement of the core produced 
by the first or regenerative quenching is not changed; the strains 
or brittleness which may have been produced in the core through 
the first quenching are relieved. In other words, by superimposing 
one quenching upon the other we have attained the desired prop- 
erties. 

Effect of Hyper-Eutectoid Zone. — The next variable is that due 
to a hyper-eutectoid zone in the case, or carburized steels which con- 
tain free cementite upon slow cooling from the temperature of car- 
burization. If we apply the treatment previously described under 
Group A the condition of this free cementite will not be affected. 
This is due to the fact that, upon heating, this free cementite is not 
dissolved by the solid solution austenite until a temperature corre- 
sponding to the Acm range (see Fig. 75) of the maximum carbon 
content of the case is attained, and which is obviously higher than 
1400° F. 

Influence of Free Cementite. — Now it has been repeatedly 
demonstrated in practice that the presence of free cementite existing, 
as it usually does, in the form of films between the grains (i.e., as a 
network), or even as spines, increases the brittleness of the case, 
interposes lines of weakness, and often results in the chipping off 
of parts of the case. (Incidentally it might be mentioned that this 



254 STEEL AND ITS HEAT TREATMENT 

is another reason for desiring a maximum carbon content in the case 
of about 0.9 per cent, when suitably adjusted and controlled methods 
of heat treatment are not used.) To eliminate this source of dan- 
ger — the free cementite — it will be necessary to heat the steel above 
the Acm range of the steel of the case in order to get this cementite 
" into solution," and to then " fix " it in that condition by quench- 
ing from that temperature. Now, provided that the maximum carbon 
content is not sufficiently high so as to raise this Acm range above the 
Ac3 range of the steel of the core, it is apparent that the treatment 
of Group B previously noted (the double quenching) will also serve 
in this instance. This treatment will likewise be applicable, with the 
above proviso, regardless of the temperature of carburization. 




s 





A 
Fig. 185.— Core of Steei Carburized at 1830° F. and Slow Cooled. (Bullens.) 

Photomicrographic Study. — The principles brought out by this 
series of treatments and their individual effect on the case and core 
may be more graphically illustrated by means of the series of photo- 
micrographs shown in Figs. 185 et seq. The steel in these photo- 
micrographs represents an ordinary low-carbon steel which has been 
cemented at 1830° F. in such a manner as to produce a carburized 
zone containing greater than 0.9 per cent, carbon. In all cases the 
steel has been allowed to cool slowly from the temperature of cemen- 
tation. 

Structure after Slow Cooling. — The micro-structure of the core 
upon slow cooling is shown in Fig. 185, it consisting of coarse ferrite 



CASE HARDENING: THERMAL TREATMENT 



255 



(light) and a small amount of coarse pearlite (dark). Similarly, the 
micro-structure of the external layers of the case is illustrated in 




Fig. 186.— Case of Steel Carburized at 1830° F., and Slow Cooled. (Bullens.) 




Fig. 187.— Core of Steel Carburized at 1830° F., Slow Cooled, and Quenched from 

1400° F. (Bullens.) 



Fig. 186, in which it is seen that the large grains of sorbitic pearlite 
are surrounded by the characteristic network structure of free 
cementite. A photomicrogaph across the entire carburized area is 
shown in Fig. 73. In other words, the steel, as a whole, exhibits the 
non-refinement characteristic of the high temperature of carburiza- 



256 



STEEL AND ITS HEAT TREATMENT 



tion, and the case is further weakened by the presence of free 
cementite. 

Effect of Lower Quenching on the Core. — If we should now 
quench the steel from about 1400° F., we see from Fig. 187 that the 
effect upon the core is to change the pearlite into martensite plus 
osmondite (the darker areas), to slightly increase it in amount (on 
account of the fact that this quenching temperature is somewhat 
above Al range), but does not give any grain refinement to the core 
as a whole. 




Fig. 188. — Case of Steel Carburized at 1830° F., Slow Cooled, and Quenched from 

1400° F. (Bullens.) 

Effect of Lower Quenching on the Case. — Similarly we see from 
Fig. 188, representing the micro-structure of the hardened high-carbon 
case, that, although the initial pearlite itself (compare with Figs. 186 
and 73) has been refined, as well as changed into hard martensite, 
the cementite network has remained unaffected. This last conse- 
quently causes the original coarse structure of the case as a whole to be 
retained (that is, unrefined), as well as the inherent brittleness due 
to this free cementite. 

Regeneration of the Core. — Now by quenching the steel from a 
temperature just above the upper critical range of the steel of the 
core, or at about 1650° F., we see from Fig. 189 that the core consists 



CASE HARDENING: THERMAL TREATMENT 257 

entirely of homogeneous martensite, and further, that the former 
coarse grain has been entirely obliterated. In other words, we have 
" regenerated " the core. 

Effect of Regenerative Quenching on the Case. — The effect of 
this same regenerative quenching upon the high-carbon case is 
shown in Fig. 190. From this photomicrograph it is evident that, 
although we have effected a rearrangement and partial solution 
of the cementite (white) and thus largely reduced the weakening 
and embrittling effect of the free cementite (as in Fig. 186), the 
heating and quenching temperature of 1650° F. has not been suffi- 
cient to dissolve entirely and " fix " the cementite in the martensite. 



W& 




Fig. 189. — Core of Steel Carburized at 1830° F., Slow Cooled, and Quenched from 

1650° F. (Bullens.) 

It is at once apparent that in this particular steel we have exceeded 
the proviso regarding the maximum carbon content which we enun- 
ciated in a previous paragraph. 

Treatment of High-carbon Case. — This leads to a consideration 
of what method we shall apply when the maximum carbon con- 
tent of the case exceeds that percentage which will cause the Acm 
range to be above the Ac3 range of the steel of the core. So in this 
particular instance we have two procedures open to us : we may either 
proceed with the quenching at 1650° F., obtain the best possible 
refinement of the core, and accept with as good grace as we can the 
presence of free cementite in the final case — granting that it is better 
distributed by this quenching than by the treatment as in Fig. 188; 
or, we may raise the temperature of the initial quenching to such 
a temperature as will completely dissolve and fix the excess cementite, 



258 



STEEL AND ITS HEAT TREATMENT 



even though it does increase the grain size (and, therefore, the brittle- 
ness) of the core. In either procedure it will, of course, be necessary 
to follow the initial quenching by the hardening quenching. The 
proposition then comes to the point as to which is the more impor- 
tant, (1) the greatest refinement of the core, or (2) the most advan- 
tageous treatment for the case, such as will give minimum brittleness, 
minimum possibility for enfoliation to occur, and the best wearing 
surface. 

Effect of Very High Quenching Temperature on the Core. — 
Assuming that the higher temperatures, which will be necessary if 




Fig. 190.— Case of Steel Carburized at 1830° F., Slow Cooled, and Quenched from 

1650° F, (Bullens.) 

the second item is to predominate, can be used without the oxidation 
of the steel (such as by the use of salt baths), excessive warping, and 
so forth, a study of the photomicrograph of Fig. 191 will aid in solving 
the problem. This figure represents the structure of 'the core after 
quenching at 1830° F., followed by a second — the " hardening " — 
quenching from 1400° F. It is at once manifest that the high quench- 
ing heat has not greatly increased the grain size of the core. And 
further, as the ferrite and sorbite have been distributed over the whole 
section in fine particles, the core should prove very tough on this 
account. That is, such treatment will generally prove satisfactory 



CASE HARDENING: THERMAL TREATMENT 



259 



for the core so long as the initial carbon content is not too high, 
and we may proceed along the lines which shall produce the best case. 
Treatment for "Best Case." — What, now, is the best case — 
in other words, the best-wearing surface; and how may it be obtained? 
From previous discussion, and from a study of the photomicro- 
graphs of this and the preceding chapter, it must be evident that the 
best-wearing surface, all things considered, is not characterized by 
the presence of free cementite as a network or as spines. Granting 
this, the way by which this condition may be avoided, assuming that 
the carburized steel contains a hyper-eutectoid zone, is first to 




Fig. 191.— Core of Steel Carburized at 1830° F., Slow Cooled, and Double 
Quenched from 1830° and 1400° F. (Bullens.) 

eliminate the free cementite by quenching above the Acm range of 
the case, and to be followed by a treatment — for purposes of grain 
refinement and hardening of the case — such as will not reproduce 
the original network condition of the free cementite. Of necessity, 
for reasons previously given, this second operation must consist of 
a quenching at about 1400° F. for straight carbon steels. 

Effect of Double Quenching on the High-carbon Case.— The 
effect of these two quenching operations is shown in the photo- 
micrograph of Fig. 192. The steel contained about 1.40 per cent, 
carbon and was quenched from about 1850° F., followed by another 
quenching at 1400° F. In this instance it will be noted that the 
free cementite appears as white dots or " spheroids " upon the darker 
martensitic groundmass, and that there is not the slightest appear- 
ance of the originally characteristic network structure of free 



260 STEEL AND ITS HEAT TREATMENT 

cementite. If the temperature of the initial quenching has not 
been sufficiently high so as to dissolve all the original network of 
free cementite, a structure will be obtained showing both spheroidal 
and network cementite. 




Fig. 192.— 1.40 per cent. Carbon Steel, Double Quenched from 1850° and 1400° F. 

X60. 

Spheroidal Cementite. — The importance of this spheroidal- 
cementite type of hyper-eutectoid structure as a wearing surface 
cannot be over-emphasized. When such a steel is first placed in 
service, the tendency will be for the martensite gradually to wear 
away, leaving the extremely hard spheroids of free cementite to take 
the wear. We then have the ideal conditions for a wearing or bear- 
ing surface: the innumerable " points " of cementite, imbedded in 
the softer and tougher martensite, act as the bearing-points; and this 
wearing surface may be ideally lubricated through the circulation 
of oil in the free zone representing the difference between the surface 
of the cementite and that of the martensite. 

Avoiding High Quenching Temperatures.— Referring again to the 
effect of high quenching temperatures upon the refinement of the 
core, it may be said that such temperatures will necessarily not bring 
out the fullest elimination of brittleness in the core. For this, as 
well as for other practical reasons involved in the obtaining of such 
high temperatures, it is advisable to avoid their use. This may be 
accomplished in two ways: by avoiding such carburizing methods 
as will necessitate their use, as will be evident from the definition 
which follows; or by a preliminary quenching directly subsequent 
to carburization which we will discuss a little later. 



CASE HARDENING: THERMAL TREATMENT 261 

Maximum Efficiency in Case-hardened Steels. — Gathering 
together some of the facts previously discussed, we will state that the 
best wearing surface, in combination with minimum brittleness of case 
(as shown by the absence of enfoliation) and of core (as shown by 
shock tests), as well as with minimum difficulties of treatment, will 
be had under the following conditions: 

(1) When the maximum carbon concentration in the case is 
greater than 0.9 per cent., but does not exceed that amount which will 
cause the temperature of the Acm range of the case to exceed the 
temperature of the A3 range of the steel of the core; and (2) when 
the following conditions subsequent to case carburizing are rigorously 
observed and their effect, with the exception of a, is at a maximum: 
(a) Slow cooling from the temperature of carburization (which we 
will shortly discuss); (6) quenching from a temperature slightly 
above the A3 range of the initial steel; followed by (c), a quenching 
from a temperature slightly above the Ac 1.2.3 range of the steel. 

Maximum Carbon Content. — Although the first statement 
regarding the maximum carbon content which we have recom- 
mended, that is, over the eutectoid ratio, is in direct contradiction 
to the opinion of many metallurgists, it is nevertheless strongly 
supported by industrial results as well as by theory. But it should 
also be distinctly noted that the conditions of the specific heat 
treatments under the second statement are strongly qualified, in 
that the best technical methods — involving accuracy and uniformity 
of heating and heat control — shall be instituted, and that the effect 
of each quenching shall be at a maximum. If such conditions can- 
not be complied with, it will be decidedly preferable to adopt such 
methods of carburization as will produce a maximum carbon content 
in the case of not much exceeding 0.9 per cent. 

Maximum Effect for Cementite Solution. — For reasons which 
will shortly be evident, it will be advisable further to amplify the 
proviso that " the effect of each quenching shall be at a maximum." 
First, then, in regard to the effect of the initial quenching upon the 
solution of the cementite. It is well known that the solution of the 
free cementite in hyper-eutectoid steels takes place very slowly. 
Due to this sluggish action, it will often be found that a heating 
of short duration slightly above the Acm range will not entirely 
dissolve the excess cementite. And further, that it is often necessary, 
in order to avoid a prolonged heating at the apparent Acm temper- 
ature, to increase this temperature to a considerable extent. Now 
when the maximum carbon content of the case is such that the 



262 STEEL AND ITS HEAT TREATMENT 

theoretic Acm temperature is considerably below the Ac3 range of the 
steel of the core, it is evident that there will be little difficulty in 
satisfactorily obtaining the full solution of this cementite. But, on 
the other hand, if the two temperatures named almost coincide, it is 
manifest that the maximum effect of the initial heating and quench- 
ing relative to the solution of the free cementite will not always be 
obtained unless such heating is prolonged. To increase the duration 
of this heating is also inadvisable, because this would tend towards 
the diffusion or equalization of the carbon content in the various 
external layers; and this, in turn, would be contrary to the purpose 
for which the high carbon was originally obtained through carburiza- 
tion. Again, quenching from a higher temperature than that origi- 
nally set would obviously exceed the provisions previously named as 
those necessary to obtain the best product, and for the present may 
be eliminated. 

Double Initial Quenching for Solution of Cementite.— It will 
therefore be inadvisable to raise the maximum carbon content of 
the case sufficiently high (through carburization) so as to bring about 
the condition of affairs which we have just been discussing. If we 
abide by the arbitrary rules which we have laid down, the only way 
out of such difficulty, if it should exist, is to double quench from the 
initial temperature. 

Relation of Initial Carbon to Maximum Carbon. — Another 
variable which should also be noted under this subject is the 
influence of the carbon content of the steel of the core upon the Ac3 
range. A study of the chart,. Fig. 75, will show that between the 
minimum carbon content used for case-hardening steels, or about 
0.05 per cent., and the maximum carbon, or about 0.25 per cent., 
there is a difference of about 125° F. This will mean a corresponding 
difference in the possible initial quenching temperature, and will, in 
turn, influence the factor of the maximum carbon content in the case 
which it is possible for us to use under these rules. This factor 
must therefore be taken into account in the method of carburizing. 
We may then say that the lower the carbon content of the steel 
to be carburized, the greater may be the maximum ,carbon content 
in the case — again assuming the previous conditions to hold. 

Double Regenerative Quenching. — Let us now consider the 
effect of the initial quenching temperature on the core. In the 
chapter dealing with case carburizing it was stated that the higher 
the temperature of carburization, and the greater the length of expo- 
sure at that temperature, the greater would be the grain size and its 



CASE HARDENING: THERMAL TREATMENT 263 

influence upon subsequent regeneration. Under such conditions 
it will not always be possible to obtain, by a single initial quenching, 
the full refinement of the core. The only alternative, in order to 
satisfy the set conditions, will be to double quench at the initial 
temperature. 

Slow Cooling after Carburization. — In a previous section we men- 
tioned that the first step subsequent to carburization was to allow 
the steel to cool slowly from the temperature of such carburization. 
When solid cements are used, the method involving the immediate 
removal of the cemented pieces from the carburizing boxes and 
throwing them into the quenching bath cannot be too strongly con- 
demned, especially if there is to be no regenerative quenching. 
In the first place, it is a practical impossibility to remove all the 
pieces from the box and to so quench them that the results will be 
identical. This statement and its logical conclusions hardly need 
further explanation. 

In the second place, in order to obtain a full refinement of the 
steel, it is absolutely necessary that the material shall be reheated 
from a temperature below the lowest critical range to a temperature 
beyond the upper critical range, for otherwise full regeneration will 
not take place. If the objects have been immediately quenched 
from a temperature near that of the carburization (i.e., without hav- 
ing been previously slow-cooled), the grain size retained by this 
quenching will be that characteristic of the highest temperature 
reached during the carburization. The grain size thus given to the 
core will be large, because the temperature of carburization must 
obviously be high if quenching is to take place before the temperature 
of the steel, during removal from the box, falls below that of the 
hardening point. If the steel should be put into service in the con- 
dition just mentioned, it would not be capable of withstanding any 
great amount of shock on account of its inherent brittleness. And 
even if the first haphazard quenching should be followed by a re- 
heating and quenching from slightly above the lowest critical range 
it is evident from previous discussion that the steel of the core as a 
whole will not be regenerated. 

In other words, if the carburization has given the proper maximum 
carbon content in the case, previously stated, such a quenching will 
be of little economic importance because it must always be followed 
by the double quenching (regenerative and hardening) necessary to 
produce maximum efficiency. Under such conditions, and for both 
theoretic and practical reasons, it is advisable to permit the car- 



264 STEEL AND ITS HEAT TREATMENT 

burized steel to cool in the boxes to a temperature at least lower than 
that of the Arl range. 

Benefits from Preliminary Quenching. — Leaving aside the 
consideration of those steels which require only a surface hardness, 
there are only two benefits which can accrue from quenching directly 
after carburization. First, there is the prevention of the " liquation " 
of the excess cementite during slow cooling, with the possible resulting 
disadvantages through enfoliation, or similarly, the liquation of the 
ferrite. The author believes that the effect of this phenomenon of 
liquation, although strongly emphasized by Giolitti, may be largely 
counteracted by the results of the effective double quenching and its 
consequent " spheroidalizing " action. The use of the preliminary 
quenching, assuming the proper maximum carbon, may be regarded 
as of indirect benefit in this first proposition. 

Second, and of particular and direct importance, is when the 
maximum carbon content of the case exceeds that amount at which 
the temperature of the Acm range is equal to, or greater than, the 
temperature of the Ac3 range of the steel of the core. Under these 
conditions the preliminary quenching — as we call it — will prevent 
the precipitation and coagulation of the excess cementite into the 
network and spines which are so difficult to redissolve during regen- 
erative heating. Consequently, this preliminary quenching will 
permit the direct use of the regenerative quenching at its proper 
temperature, even though the carbon content of the case is higher 
than the governing ratio between Acm and Ac3 and which, under 
conditions of slow cooling, would demand the use of a higher regenera- 
tive quenching. It is manifest, however, that such preliminary 
quenching, to be effective, must take place at a temperature higher 
than the specific Acm temperature, or at about that of the cementa- 
tion proper. 

Use of Salt-bath Heating. — Before summing up the treatments 
given in the foregoing pages, there are three points of practical inter- 
est which should be noted. The first of these has to do with the 
method of heating the steel for quenching. It is obvious that oxida- 
tion, even of very slight amount, must be entirely prevented. The 
best and surest method of attaining this is by the use of molten baths. 
Of these, the salt baths are to be preferred to the use of lead, at least 
for temperatures over 1500° F., on account of the poisonous fumes of 
the latter at the high temperatures. 

Interrupted Regenerative Quenching. — The second item refers 
to the regenerative quenching. On account of the tendency of the 



CASE HARDENING: THERMAL TREATMENT 265 

high-carbon steels to check or crack when high quenching tempera- 
tures are used, it is advisable to remove the steel from the water 
bath when its red color is seen to disappear. As the steel " loses its 
color " at a temperature under that of the lowest critical range — 
that is, below that temperature at which the transformation in cool- 
ing is totally effected, it is evident that this interrupted cooling will 
in no wise affect the regeneration of the core. Its influence upon the 
structure of the case will ■ also have little practical importance, 
primarily because it is not desired through this quenching to obtain 
a maximum hardness; and further, because there will be little or no 
tendency for any excess cementite to precipitate as a network struc- 
ture. If any of the excess cementite should be thrown out of solu- 
tion, it is more apt to be of the spheroidal type. Whether or not this 
cooling is interrupted at about 900° F., it is always advisable to 
remove the steel from the bath before it has become entirely cold. 

Coagulation of Cementite. — In the third place, we would refer 
briefly to the " hardening " or second quenching. If the case con- 
tains greater than the eutectoid ratio of carbon, the duration of the 
heating at this lower temperature should not be prolonged over a 
greater period than is necessary thoroughly and uniformly to heat 
the case to the proper hardening temperature. A prolonged heating 
would have the tendency to coagulate the cementite which is ordi- 
narily precipitated at this temperature, thus opposing the realiza- 
tion of the conditions of maximum effectiveness. 

Summary. — We may sum up the general situation, and give to 
each class of steel the treatment which we recommend to obtain the 
" best wearing surface," combined with minimum brittleness of case 
and core. 

Classification of Case-carburized Steels 

Group A. Steels case carburized at temperatures approximating 

that of the upper critical range of the initial steel. 
Group B. Steels case carburized at temperatures considerably exceed- 
ing that of the upper critical range of the initial steel. 
Class 1. Maximum carbon content of the case does not exceed 

0.9 per cent. 
Class 2. Maximum carbon content of the case greater than 
0.9 per cent., but is less than when Acm of the case 
equals Ac3 of the core. 
Class 3. Maximum carbon content of the case greater than 
that specified under (2) . 



266 STEEL AND ITS HEAT TREATMENT 

Classification of Treatments for Specific Steels 

Group A. Class 1. Treatment I. 

2. II. 

3. III. or IV. 

Group B. Class 1. Treatment II. 

2. II. 

3. III. or IV. 
Treatment I. 

a. Cool slowly. 

b. Quench from slightly over the Acl.2.3, or about 1400° F. 

Treatment II. 

a. Cool slowly. 

b. Quench from slightly over Ac3 of the core. Dependent upon 

the carbon content, this will vary from 1650° to about 
1525° F. 

c. Quench from slightly over Acl.2.3, or about 1400° F. 

Treatment III. 

a. Quench directly subsequent to carburization, without slow 

cooling, from at or near the temperature of carburization 
but not lower than Acm. Dependent upon the carbon 
content of the case, Acm will vary from about 1650° F. for 
1.20 per cent, carbon (or thereabouts), to about 1800° for 
1.45 per cent, carbon. It is not advisable to quench at a 
temperature higher than 1800° F. 

b. Treatment as in II, b and c. 

Treatment IV. 

a. Cool slowly. 

b. Quench from a temperature over the Acm, dependent upon the 

carbon content of the case. (See III, a.) 

c. Quench from slightly over Acl.2.3, or about 1400° F. 

Note : This treatment requires a slight sacrifice in the minimum 
brittleness of core in order to obtain " best wearing surface." 

Mechanical Effects of Treatments. — The effect upon the mechan- 
ical properties of the case and core of various treatments is given in 
the following table taken from Guillet. The steel used was of the 
ordinary type for case hardening, classed as " extra soft." 



CASE HARDENING: THERMAL TREATMENT 



267 



Treatment. 



Non-cemented steel, heated at 1700° F. and cooled 
in air 

Non-cemented steel, quenched at 1700° F. in water 

Steel cemented at 1830° F. for 0.047 in. and cooled 
slowly 

Same cementation; quenched at 1830° F. in water. 

Same cementation; quenched twice in water, at 
1830° and 1375° F 



Surface Hard- 
ness of the 
Case, Shore 
Method. 




38.5 
79.8 



Further Treatments not giving Maximum Efficiency. — Case- 
hardened objects having a comparatively thin cemented zone 
(ys m - or l ess ) ma Y broadly be divided into those articles which 
require only surface hardness and work under fairly uniform pressure 
without shock, and those articles which must withstand shock, 
bending strains, etc. We have discussed at some length both the 
carburization and the heat treatment which are required by those 
of the latter class. The heat treatment of those articles of the first 
class we have previously referred to, but for purposes of summarizing 
we may divide it as follows; 

Treatment V. 

a. Quench directly after carburization (without slow cooling), 

but at a temperature not less than 1350° F., or that of Arl. 

The results may be varied over a wide range according to the 

temperature of quenching. 

Note : This treatment is for those articles which merely demand 

a hard surface, and in which brittleness and en foliation may not be 

considered. 



Treatment VI. 

a. Cool slowly. 

b. Quench from slightly over Ac3 of the initial steel, varying from 

1650° to 1525° according to the carbon. 
Note: This treatment is for those articles which demand a 
tough core and a comparatively hard surface — that is, the elimination 
of brittleness in the core is of more importance than maximum 
surface hardness. 



268 STEEL AND ITS HEAT TREATMENT 

Treatment VII. (Similar to Treatment I.) 

a. Cool slowly. 

b. Quench from about 1400° F., or slightly over Acl. 

Note : This treatment is for articles which demand a maximum 
surface hardness, or as much as can be obtained from a single quench- 
ing, without reference to the brittleness of core or to the dangers 
of enfoliation through the presence of free cementite. With low 
temperatures of carburization and with a carbon maximum of 0.9 
per cent, this classification would of course correspond to Group A, 
Class 1. 

Alloy Steels. — The treatment of alloy steels will be considered 
under their respective chapters. In the main, however, the theory 
of treatment does not vary, although the actual temperatures may 
be changed on account of the influence of certain alloys upon the 
position of the critical ranges. 



CHAPTER XII 



CARBON STEELS 



Foreword. — In this and in the following chapters on various 
steels there will be given the physical results which are representative 
of the different steels and their treatment. Such results have been 
gathered from practical work and experiment, and although the 
results of various treatments will vary according to the individual 
steel and the personal equation of the operator, they may be consid- 
ered as fairly representative of the steel and treatments given. 

Further, it must be remembered that the size of section or mass 
of the steel has a very important influence upon the physical test 
results. The same results will not be obtained in a steel bar of 
4 ins. diameter as in a bar of the same steel with similar treatment 
and of only 1^ ins. diameter. Similarly, different results will be 
obtained near the outer surface of a large forging in comparison 
with a test taken near the center. 

As an example of the effect of the size of piece upon the tensile 
strength, under the same treatment, we may cite the following 
examples : 



Diameter 
of Bar 
Inches. 


Tensile Strength. 
Lbs. per Sq. In. 


1 

2 
1 

H 

2 
2 1 
3 
3i 


137,000 
132,000 
127,000 
122,000 
113,000 
105,000 
100,000 



Hardness vs. Maximum Strength. — The following equations 
connecting maximum strength, Brinell hardness number and sclero- 
scope hardness number have been computed x from several hundred 

1 R. R. Abbott, A. S. T. M., Vol. XV, Part II, 1915, p. 43 et seq. 

269 



270 



STEEL AND ITS HEAT TREATMENT 



tests made with carbon steels of different carbon content and heat 
treated to bring out all possible physical properties: 

(1) M = 0.73 5-28. 

(2) M=4A S-28. 

(3) 5 = 5.6 £+14. 

M = maximum strength in units of 1000 lbs. per sq. in. 
5 = the Brinell hardness number. 
$ = the scleroscope hardness number. 

The maximum strength corresponding to different Brinell 
values as determined by equation (1) for carbon steels is as follows: 



Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


1C0 
150 
200 
250 
300 


45,000 

81,000 

118,000 

154,000 

191,000 


350 
400 
450 
500 
550 


227,000 
264,000 
300,000 
337,000 
373,000 



The maximum strength corresponding to different scleroscope 
values as determined by equation (2), and the corresponding Brin- 
ell numbers as determined by equation (3), for carbon steels, are 
as follows: 



Scleroscope. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


20 
30 
40 
50 
60 
70 
80 
90 
100 


60,000 
104,000 
148,000 
192,000 
236,000 
280,000 
324,000 
368,000 
412,000 


126 
182 
238 
294 
350 
406 
462 
518 
574 



VEKY LOW CARBON STEELS: UNDER 0.15 CARBON 

The " dead soft " steels, about 0.10 per cent, carbon, find but 
little application to heat-treatment purposes. Contrary to general 
opinion, these steels do respond to heat treatment, although, of 
course, to a very limited extent. The best treatment to which these 
steels can be subjected is a quenching from about 1550° to 1600° F., 



CARBON STEELS 



271 



with or without reheating, the reheating being omitted when con- 
ditions (of strain caused by quenching) will permit. Such a 
treatment will refine the grain and remove any strains set up by 
previous working; will confer added toughness; and will put the 
steel in the best condition for machining. This last is an important 
point, for this very low carbon steel, without high manganese and 
phosphorus, and in either the annealed or toughened condition, often 
does not machine freely, but is apt to tear badly in threading and 
turning operations. The heat treatment of very low carbon steel 
as applied to the wire industry is discussed in a subsequent chapter. 

Annealed. — For annealing 0.1 per cent, steel, heat as rapidly as 
consistent with the size and shape of the piece to a temperature 
slightly above the upper critical range of the steel, approximately 
1600° F. In these very low carbon steels the change into austenite 
takes place very rapidly, so that it is only necessary to allow the heat 
to penetrate the steel at the temperature noted above. As the 
grain begins to coarsen rapidly with increase in temperature and 
length of time held there, care should be taken in not overheating 
nor maintaining the annealing temperature for too long a time. 
Cooling may be carried out comparatively rapidly without danger 
of hardening the steel. The annealing of these very low carbon 
steels is usually for the purpose of relieving such strains as may be 
incurred by cold crystallization or by previous heating at a low- 
red heat for any great length of time. 

Heat Treated. — The results obtained from the treatment of test 
bars f ins. in thickness of acid open-hearth steel of the composition: 

Per Cent. 

Carbon 0.10 

Manganese 0.32 

Phosphorus 0.028 

Sulphur 0.024 

Silicon 0.019 

are given in the following table: 



Steel. 



Tensile 
Strength. 
Lbs. per 

Sq. In, 



Elastic 

Limit . 

Lbs. per 

Sq. In. 



Elongation. 

Per Cent. 

in 8 Ins. 



Reduction 

of Area. 

Per Cent. 



As rolled 

Annealed 

Water quenched from 1575° F. . 
1575° F. Water/1300 F 



51,625 
48,800 
64,720 
53,055 



35,500 
31,770 
45,550 
36,500 



34.5 
37.5 

22.8 
35.35 



65.3 
67.5 
61.15 
66.05 



272 



STEEL AND ITS HEAT TREATMENT 



General Specification, Annealed 



Tensile Strength. 
Lbs. per Sq. In. 


Elastic Limit. 
Lbs. per Sq. In. 


Elongation. 
Per Cent, in 2 Ins. 


Reduction, 
of Area. 
Per Cent. 


45,000 

to 
55,000 


28,000 

to 
36,000 


40 
to 
30 


65 

to 
55 



0.15-0.25 CARBON STEEL 



This grade of straight carbon steel is generally known to the trade 
as " machinery steel/' and as such has innumerable uses where 
strength is not an all-important factor. The steel forges and ma- 
chines well. The lower carbons find their greatest application in 
the case-hardening processes which have been previously described. 
The higher carbons are used considerably in certain engine forg- 
ings such as tie rods, valve stems, nuts, flanges, pins, levers, etc.; 
for machine work of various description; for structural purposes 
in automobile construction, etc. 

Heat Treated. — Heat treatment of the lower carbons of this 
range confers but little additional strength except in thin sections, 
but does have a most desirable influence in the refinement of grain 
after forging or other elaboration. Hardening should be done 
from a temperature exceeding the upper critical range — which is 
about 1550° F. for 0.15 per cent, carbon, and about 1525° F. for 
0.20 per cent, carbon — in order to effect the full absorption and 
diffusion of the excess ferrite. Some engineers recommend quench- 
ing at 1650° F. or even higher, but the author believes that such 
high temperatures are not only detrimental on account of a greater 
tendency to warping, oxidation and higher cost of treatment, but 
are also unnecessary metallurgically. In other words, those tem- 
peratures should be used which will produce the most efficient com- 
bination of physical properties, refinement of grain arid low cost of 
production. From the results of extensive research work upon 
0.18 to 0.28 carbon stock used for automobile purposes, and from a 
study of its working out in practice, the author recommends a 
quenching temperature of about 1500° to 1525° F. for these steels. 
Temperatures lower than 1500° do not bring out the full effect 
of the treatment, as is shown by the following average results (from 
a large number of tests) upon the same steel; 



CARBON STEELS 



273 



Quenched in Oil 
from — ° F.; Re- 
heated to 800° F. 


Tensile Strength. 
Lbs. per Sq. In. 


Elastic Limit. 
Lbs. per Sq. In. 


Elongation. 

Per Cent in 

2 Ins. 


Reduction 
of Area. 
Per Cent. 


1450 
1500 


70,220 
79,590 


43,460 
52,500 


24.1 
25.6 


48.4 
52.6 



with hardening temperatures higher than 1550° F. there is 
practically no increase in the physical properties worthy of men- 
tion, and, moreover, the structure then begins to coarsen rap- 
idly. The microscope shows little or none of the original struc- 
ture when the steel has been quenched from about 1500° to 
1525° F. 

With carbons greater than 0.18 or 0.20 per cent., and particu- 
larly if the section is small, or the manganese content is more than 
0.60 per cent., the necessity of reheating or toughening after quench- 
ing becomes apparent. Hardening small sections, such as are used 
in automobile construction, from about 1525° F. without subse- 
quent drawing — especially if water has been used as the cooling 
medium — will produce an inherently brittle steel. The physical 
characteristics under these conditions will be approximately as 
follows : 

Tensile strength, lbs. per sq. in 90,000 to 110,000 

Elastic limit, lbs. per sq. in 60,000 to 75,000 

Elongation, per cent, in 2 ins 17 to 12 

Reduction of area, per cent 30 to 15 

By reheating to 800° or 900° F. a considerable increase in tougn- 
ness and ductility is obtained, approximating: 

Tensile strength, lbs. per sq. in 70,000 to 85,000 

Elastic limit, lbs. per sq. in 45,000 to 60,000 

Elongation, per cent, in 2 ins 35 to 20 

Reduction of area, per cent 65 to 45 



Cold-rolled material, subsequently given the same heat treatment 
as hot-rolled material of the same chemical composition, will 
usually show about 8000 to 10,000 lbs. per square inch higher in 
elastic limit and tensile strength. 

Characteristic results from commercial work are given in the 
following table: 



274 



STEEL AND ITS HEAT TREATMENT 



Material. 


p 
o 

M 

o 


C 


Quenched 

in Oil from 

°F. 


Re- 
heated 
to °F. 


Tensile 
Strength. 
Lbs. per 

Sq. In. 


Elastic 

Limit. 

Lbs. per 

Sq. In. 


Elon- 
gation. 
Per 

Cent 
in 2 In. 


Reduc- 
tion of 
Area. 
Per 
Cent. 


General char- 

acteristics 


0.18 

to 
0.25 


0.40 

to 

0.80 


1500 

to 
1550 


800 
to 
900 


70,000 

to 
85,000 


45,000 

to 
60,000 


35 
to 
25 


65 
to 
45 


Auto, lever 


0.18 


0.40 


1650 


800 


70,030 


45,400 


32 


64 


Pressed auto, frame. 


0.22 


0.40 


1530 


800 


71,950 


43,400 


29 


56 


Engine forging 


26 


0.28 


1650 


1025 


77,210 


52,200 


28 


65 


Old rolled M in. plate 


0.24 


0.60 


1525 


900 


93,300 


65,250 


20.5 


51 



The above remarks apply mainly to the smaller sections up 
to 2 ins. in thickness, but are nevertheless applicable in part to 
heavy work. With the increase in sectional area, the effect of hard- 
ening decreases, and for particularly heavy work may result only 
in a refinement of grain. Thus, for heavy, oil-treated forgings, 
toughening may not be considered a necessity; such reheating will, 
however, relieve the strains which are always inherent to quenched 
steels. Large forgings thus treated will show an elastic limit of 
30,000 to 50,000 lbs. per square inch, with an elongation of 35 to 
25 per cent, in 2 ins. 

Annealed. — There is probably more disagreement and argument 
as to the proper annealing temperatures for this range of carbon 
steel than for any other. Opinion and practice are divided over the 
use of a comparatively high temperature — 50° to 100° over the 
upper critical range — or a lower temperature laying somewhere 
between the Acl and Ac3 ranges. In this group the Acl and Ac3 
ranges are widely separated and the influence of the carbon-mangan- 
ese content is rapidly increasing. The high annealing temperature, 
1550° to 1600° F. or more, will give ample opportunity for the 
absorption of the excess ferrite, for diffusion and for equalization. 
On the other hand, there is according to some authorities a marked 
increase in grain size from 1350° or 1375° F. and upwards. 

The whole question really depends upon the condition of the steel 
before annealing. If the " breaking-down " during elaboration — 
either rolling or forging— has been severe, if high temperatures have 
been used, and if the finishing temperature has not been just right, 
a high annealing temperature may be necessary to entirely relieve 
the strains and equalize the steel. On the other hand, if the steel 



CARBON STEELS 



275 



has been carefully worked and the micrographic structure is fairly 
good, the lower temperatures will probably be entirely satisfactory. 
Much must be left to the operator and his own particular problem. 
The main point to bear in mind is that the lowest temperature 
should be used which will produce the desired results. 

If we assume as average figures for annealed steel of this cer- 
bon range : 

Tensile strength, lbs. per sq. in 58,000 to 65,000 

Elastic limit, lbs. per sq. in 28,000 to 35,000 

Elongation in 2 ins., per cent over 30 

and compare these with the results of a tensile test taken from 
the steel to be annealed, a very good idea of the degree and length 
of heating may be obtained. For example, the following results 
from 1^-in. rounds for gun barrels show that a high annealing tem- 
perature was not necessary in this case, inasmuch as the original 
steel was in excellent condition. 

Gun barrel steel, l|-in. rounds. 
Carbon, 0.18 per cent. 
Manganese, 0.50 per cent. 
Phosphorus, 0.070 per cent. 
Sulphur, 0.055 per cent. 
Silicon, 0.055 per cent. 







Tensile 


Elastic 


Elongation. 


Reduction 


Treatment. 


Strength. 


Limit. 


Per Cent. 


of Area. 






Lbs. per Sq. In. 


Lbs. per Sq. In. 


In 3 Ins. 


Per Cent. 


As Rolled 




66,750 


33,820 


33.3 


57.6 


Annealed at 












degrees F. 


for minutes 










1360-1400 


30 


64,960 


34,050 


38.0 


61.0 


1500 


20 


65,180 


32,930 


38.3 


58.3 


1500 


105 


64,060 


33,150 


39.1 


62.3 


1830 


15 


62,940 


31,810 


35.7 


56.3 


2120 


5 


61,150 


31,580 


33.8 


53.1 



On the other hand, the following cold -rolled automobile-frame steel 
was particularly " hard " before annealing and required a tempera- 
ture of 1550° F. to relieve thoroughly the effect of the cold work: 

Carbon, 0.24 per cent. 
Manganese, 0.38 per cent. 
Phosphorus, 0.028 per cent. 
Sulphur, 0.038 per cent. 



276 



STEEL AND ITS HEAT TREATMENT 



Tensile 

Strength. 
Lbs. per Sq. In. 



Elastic 

Limit. 

Lbs. per Sq. In. 



Elongation. 
Per Cent. 
■ in 2 Ins. 



Before annealing 

After annealing at 1550° F. 



100,400 
66,000 



68,500 
38,100 



18.6 
37.0 



For the average run of annealing work for this range of carbon, 
a temperature of about 1500° F. will be found to give satisfactory 
results; individual cases must be treated as such. 



0.25-0.35 CARBON STEEL 

Steel containing from 0.25 to 0.35 per cent, carbon is known 
as soft-forging steel and is used principally for structural purposes 
in infinite variety. It responds in a most satisfactory manner to 
welding, forging and machining, and may be vastly improved by 
proper heat treatment. Under skillful treatment, the variety of 
combinations of strength and ductility are to be had in probably 
no other range of carbons. 

Relative to static strength, some really wonderful results — for 
straight carbon steels — in the way of high tensile strength with high 
ductility have been obtained from heat-treated (oil quenched and 
toughened) forgings of 0.30 to 0.35 per cent, carbon. The follow- 
ing results, obtained from the center of a 5-in. electric car, heat- 
treated axle, the axle being selected at random from a group of 
about one hundred forgings, give an idea of the extent to which 
proper heat treatment may develop the physical properties: 

Electric Car Axle, 0:32 Carbon, Acid Steel. 

Tensile strength, lbs. per sq. in 91,700 

Elastic limit, lbs. per sq. in 61,620 

Elongation, per cent, in 2 ins 33.5 

Reduction of area, per cent 48.1 

In the hardened condition — without subsequent tempering — 
these steels may be used for gears. In the toughened condition 
these steels present the maximum resistance to fatigue and other 
dynamic stresses, as represented by alternating impact and other 
tests, over any of the straight carbon steels; the dynamic strength 
probably apexes at about 0.30 per cent, carbon, as far as the author 
can judge from his own researches and from the work and conclusions 
of others. 



CARBON STEELS 277 

Untreated. — In the untreated condition, with standard man- 
ganese, phosphorus and sulphur, the average tensile strength of 
these steels will be about as follows: 

Carbon. Acid Steel. Basic Steel. 

0.25 to 0.30 67,000 to 78,000 63,000 to 72,000 

0.30 to 0.35 69,000 to 83,000 65,000 to 74,000 

Rolled plates, from 2 to 4 ins. thick, made of basic steel with 0.25 
to 0.35 per cent, carbon and about 0.40 per cent, manganese, will 
usually fulfill the following specifications : 

[Tensile strength, lbs. per sq. in 65,000 to 75,000 

Elastic limit, lbs. per sq. in 33,000 to 37,000 

Elongation, per cent, in 2 ins 30 to 25 

Reduction of area, per cent 50 to 36 

These results may also be considered as generally applicable to 
untreated steel of this analysis, but which has had more or less 
elaboration or working. 

Heat Treated. — The upper critical range decreases from about 
1500° F. for 0.25 per cent, carbon, to about 1425° F. for the 0.35 
per cent, carbon steel. Practical experience has shown that a 
quenching temperature of 1500° to 1525° F. for the lower carbons of 
this range, and 1450° to 1500° F. for the higher carbons will give 
satisfactory results under ordinary conditions. If the heating has 
been conducted uniformly and not too rapidly — especially when 
approaching the maximum temperature — the original structure of 
the steel should be entirely eliminated, as the temperatures recom- 
mended are distinctly above the upper critical range. Never- 
theless, some metallurgists prefer to quench these steels from a 
higher temperature, say 1575° to 1600° F., in order to make cer- 
tain of the complete change in structure and to obtain a maximum 
hardening effect. In either case, intelligent furnace operation and 
heat control will probably be the governing factor rather than the 
indicated furnace temperature or mere theorizing. 

For forgings in which especially high qualities are desired, double 
quenching will produce a refinement of grain and correspondingly 
higher elastic limit and ductility than are usually obtained by the 
single treatment. The temperatures recommended for this range 
of carbons are: 

1. First quenching [from 1600° F., or from 1500° to 1550° F. 
if the higher quenching should prove too drastic. 



278 



STEEL AND ITS HEAT TREATMENT 



2. Second quenching from 1425° to 1450° F., followed by 

3. Suitable toughening according to the size of piece and 

physical properties desired. 
The results to be obtained from heat treatment will vary largely 
for this range of carbon in particular, due to such influence as the 
increase of a few points in the carbon content (particularly noticeable 
in these mild steels), the size of the section, the quenching medium, 
and so forth. The results given under the 0.15 to 0.25 carbon range, 
and under the 0.35 to 0.45 carbon range to follow, may be used as a 
general measure of the carbons under discussion. Stated roughly, 
these carbons will give elastic limits ranging from 35,000 to 80,000 




Fig. 193.— 0.28 per cent. Carbon Steel. X30. (Campbell.) 



lbs. per square inch, with corresponding elongations of 30 to 10 per 
cent, in 2 ins. 

Annealed. — As has been previously explained, heating for anneal- 
ing to just above the Acl (lower) critical range will refine the ground- 
mass only, while complete refinement is shown by the disappearance 
of the ferrite and network beyond the upper critical range (Ac3). 
As an example of this, examine the photomicrographs of a basic 
open-hearth steel containing 0.28 per cent, carbon and 0.52 per cent, 
manganese, as "shown in Figs. 193, 194, and 195. The first photo- 
graph shows the original steel with its coarse, weak structure. Fig. 
194 shows the same steel annealed at 1425° F., or just over the Acl 
range; the perlitic ground-mass has been entirely refined, but there 
still remains the unabsorbed and undiffused excess ferrite. Fig. 



CARBON STEELS 



279 



195 shows the same steel heated to 1520° F. and slow cooled in the 
same manner; but in this case the structure has been entirely changed 
and refined by heating to a temperature over the upper critical range. 




Fig. 194.— 0.28 per cent. Carbon Steel Annealed at 1425° F. 
X39. (Campbell.) 




Fig. 195.— 0.28 per cent. Carbon Steel Annealed at 1520° F. 
X39. (Campbell.) 



Practical experience has shown that a temperature of 1500° to 
1525° F. will give excellent results for the full annealing of steels 
within this range of carbons. On account of the hardening effect 
of air cooling steels with over 0.20 per cent, carbon when in small 



280 STEEL AND ITS HEAT TREATMENT 

sections, these steels should be slow cooled, either in the furnace, 
in lime or in ashes. 

In regard to the physical properties to be obtained from the 
annealing of these steels, the lower carbons of this range should 
always meet tthe U. S. Government specification of: 

Tensile strength, lbs. per sq. in 60,000 

Elastic limit, lbs. per sq. in 30,000 

Elongation, per cent, in 2 ins 30 

while the higher carbons will usually give : 

Elastic limit, lbs. per sq. in 35,000 to 45,000 

Elongation, per cent, in 2 ins 22 to 32 

Reduction of area, per cent 30 to 60 

0.35-0.45 CARBON STEEL 

Straight carbon steels with 0.35 to 0.45 per cent, carbon are 
particularly suited to medium and heavy forgings for which the 
lower carbons would not give sufficient strength, and for which it 
is also not desirable to use water quenching on account of the possi- 
bility of starting incipient cracks or strains. This steel is commonly 
used for high-duty and moving machine parts; for axles, side bars, 
crankpins and other locomotive forgings; for guns and gun forgings; 
for crank shafts, driving shafts and similar automobile parts; and 
for general structural purposes requiring the combination of maxi- 
mum strength with minimum brittleness. It has excellent dynamic 
strength, although probably not quite so much as the previous 
class of 0.25-0.35 carbon. Steel with 0.40 carbon according to 
Robin 1 presents the greatest resistance to abrasive action (wear) . 
These steels are easy to machine when in the annealed or soft- 
toughened condition, but should not be used for screw machine stock. 

The upper critical range temperature of this steel is about 1425° F. 
to 1400° F. 

Untreated. — The average untreated American open-hearth steel 
with standard manganese, phosphorus and sulphur will average 
about as follows in tensile strength: 

Carbon. Acid Steel. Basic Steel. 

0.35 to 0.40 78,000 to 92,000 70,000 to 78,000 
0.40 to 0.45 87,000 to 100,000 76,000 to 89,000 

1 J. Robin, Inst. Journ., II, 1910. 



CARBON STEELS 



281 



Annealed. 



Remarks. 


C. 


Mn. 


Phos. 


Sul. 


Tensile 
Strength. 
Lbs. per 

Sq. In. 


Elastic 

LirMt. 

Lbs. per 

Sq. In. 


Elong- 
ation. 
% in 
2 Ins. 


Red. of 

Area. 

Per 

Cent. 


General limits. . . . 


0.35 

to 
0.45 


not 

over 

0.70 


under 
0.045 


under 
0.045 


70,000 

to 
85,000 


38,000 

to 
50,000 


28 
to 
20 


55 

to 
40 


Forged gun jacket 
acid steel 


0.35 


0.25 


0.038 


0.019 


77,080 


39,500 


27 




Forged gun jacket 
basic steel 


0.43 


0.22 


tr. 


0.023 


78,180 


43,100 


25.5 




8-in. axle acid 
steel annealed 
at 1400° F 


0.42 


0.51 






78,420 


47,460 


28 


54.5 



Heat-treated. — Large sections, when quenched in good mineral oil 
from 1400° to 1500° F., and toughened at 900° to 1200° F. (according 
to the carbon content and largest section), should always meet the 
specification of 85,000—50,000—22—45. The following tests taken 
from large forgings show the variety of combinations of strength 
and ductility which may be obtained: 



Forging. 



Gun jacket 

Axle 

Gun jacket 
Shaft 



Carbon. 



0.35 
0.41 
0.43 
0.42 



Treatment. 



1500-O/1200 
1450-w/l000 
1500- o/l200 
1525- o/l 300 



Tensile 
Strength. 
Lbs. per 

Sq. In. 



109,560 
90,250 

111,100 
82,040 



Elastic 

Limit. 

Lbs. per 

Sq. In. 



65,090 
54,575 
69,700 
57,060 



Elonga- 

tino. 

Per Cent. 

in 2 Ins. 



16.5 
25.4 
17.0 
29.0 



Red. of 

Area. 

Per Cent. 



52.4 
55.0 



o = oil. w= water. 



It is always advisable to keep the drawing temperature as near 
1200° to 1250° F. as possible, not only because it is easier for the 
furnace operator to obtain more accurate temperature control at 
these more readily distinguished " reds," but also on account of the 
greater dynamic strength which is obtained by the use of the higher 
drawing temperatures. 

The results obtained from the water quenching from 1450° F. 
and subsequent toughening of small rounds of 0.40 per cent, carbon 
steel are given in the chart in Fig. 196. 



282 



STEEL AND ITS HEAT TREATMENT 





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CARBON STEELS 283 

LOCOMOTIVE AXLES 

Locomotive axles and other heavy forgings used in locomotive 
construction are illustrative of the treatment of pieces of large 
section of the above carbon range. 

Heat-treated Axles. — For heat-treated axles the carbon content 
will range between 0.35 and 0.50 per cent., and with such steel 
the treatment may generally be adjusted to meet the standard 
specification of: 

Tensile strength 85,000 lbs. per sq. in. 

Elastic limit 50,000 lbs. per sq. in. 

Elongation 22 per cent, in 2 ins. 

Reduction of area 45 per cent. 

The quenching of the axles, usually from temperatures of 1400° 
to 1500° F., is mainly a proposition of correct heat application and 
efficient handling. Both oil and water are used extensively for 
hardening axles. Water will bring out the full effect of heat treat- 
ment by giving the highest tensile test properties of which the steel 
is capable; with the same ductility, oil quenching will give lower 
tensile values than water quenching. A steel with lower carbon 
content may more logically be used with water quenching than with 
oil. Water requires no expensive cooling nor circulation system, 
and has practically no cost of upkeep or replenishment, as all 
these may be regulated by the intake of fresh, cold water. On 
the other hand, many engineers severely condemn the use of water 
in that it is too harsh in its action upon large masses of steel — as in 
axles, that cracks are more liable to develop, and that internal 
strains are set up which often are not always entirely relieved by 
the reheating or toughening. Oil is much the safer quenching 
medium to use for axles, will give more uniform results, and should 
only be replaced by water for economic reasons. 

The hardened axles are charged while still warm into the re- 
heating furnace, which is maintained at such temperature as will 
relieve all strains set up in the hardening and at the same time 
give the physical properties desired. This temperature will vary 
between 900° and 1200° F., depending upon the chemical composi- 
tion of the steel. The higher the drawing temperature, the more 
ductile the steel and apparent coarseness of the grain, due to the 
transformation of the transition constituents troostite and sor- 
bite into pearlite plus free ferrite. Straight carbon steels quenched 
in water and drawn at 1000° or 1100° F. will be entirely sorbitic, but 



284 STEEL AND ITS HEAT TREATMENT 

at 1200° may show considerable free ferrite. Too much emphasis 
cannot be given to the necessity of keeping a uniform temperature 
and allowing sufficient time for the heat thoroughly to penetrate the 
axle. It is then preferable for the axles to cool with the furnace, 
rather than to remove them while still at the toughening temperature. 
The temperatures used by one manufacturer of acid steel axles 
and other large forgings to meet the standard A. S. T. M. specifica- 
tions are as follows: after quenching in oil from 1450° F., reheat as 
below and air-cool or cool in ashes. 

0.42 to 0.45 per cent, carbon 1175° F. 

0.38 to 0.42 " " " 1125° F. 

0.33 to 0.38 " " " 1075° F. 

0.28 to 0.33 " " " 1000° F. 

The following are characteristic tests from Open-hearth steel 
locomotive axles (Penna. R.R.) : 



0.41 Carbon 



Fore-pfi Treated, 

Dorgea. 1500 water/ 1000° F. 



Tensile strength 73,627 90,250 

Elastic limit 31,505 54,575 

Elongation 31.6 25.4 

Reduction of area 43.6 52.4 

0.50 Carbon 

Forged. 1500 <, wate r/1200 o F. 

Tensile strength 83,430 90,092 

Elastic limit 34,370 53,655 

Elongation . 22.6 27. 1 

Reduction of area 30 . 52 . 8 

" Tempered " Axles. — For want of a better name, under this 
heading might be included such as are treated by the " Coffin " or 
similar processes. The main principle consists in heating the axle as 
usual for hardening, then immersing in the quenching bath for a cal- 
culated number of seconds, immediately withdrawing, and allowing 
the heat from the interior of the axle to " temper " the part which 
had been hardened by the short immersion in the oil or water. 
This process has been developed to such a nicety that surprisingly 
uniform results may be obtained if the test is always taken from the 
same relative place, such as half-way between the center and the 
outside, The main arguments for the process are that it is simple 



CARBON STEELS 



285 



and that the axle will have a tough (annealed) core, and, at the same 
time, a hard wearing surface. Non-uniformity of structure is the 
principal argument of those condemning the process, and on account 
of the inevitable " human equation " which enters into it, would 
seem to be not without justice in many instances. 

Annealed Axles. — The axles are heated up slowly and uniformly 
to a temperature slightly in excess of the upper critical range, main- 
tained at this temperature for sufficient time for the steel to respond 
to the heat, and then cooled with the furnace. If the working 
and the finishing temperature during forging have been adjusted 
so as to give a fine grain to the steel, besides good physical test re- 
sults, it will be found that heating to a temperature over the critical 
range may not be necessary in many instances. Where it is neces- 
sary to anneal large numbers of heavy axles, the hot axles may be 
removed quickly to a pit and covered with lime or ashes. Annealing 
alone will generally overcome the strains set up in the previous proc- 
esses of manufacture, but it does not bring out the higher physical 
properties of which the steel is capable. Annealed axles will show 
pearlite and free ferrite, the apparent size of the ferrite grains de- 
pending upon the rate of cooling and the time thus given for the 
ferrite to separate out from the matrix. 

In order to obtain the required tensile strength upon anneal- 
ing it is necessary to use a steel of higher carbon content than that 
used for full heat treatment. A tensile strength of 80,000 lbs. per 
square inch will require a 0.50 carbon steel or higher. A temperature 
of 1500° F. is generally recommended (A. S. T. M.) for annealing 
0.40 to 0.60 per cent, carbon steel, but since the critical range of 
this steel is about 1400° F. or a little under, an annealing temperature 
of 1400° to 1450° F. will give a better fracture, together with a better 
combination of tensile strength and ductility. 

Much better results will be obtained with the lower temper- 
ature, although the time required for the annealing generally is longer. 
The following results from acid open-hearth steel will show the 
effect of the lower temperature anneal : 



Heat 4261 


Annealed at 1400° 


Carbon, 0.42 per cent 

Manganese, 0.51 per cent. . . 
Phosphorus, 0.034 per cent . . 
Sulphur 0.028 per cent 


Tensile strength 

Elastic limit 

Elongation in 2 ins . . . 
Reduction of area .... 


78,420 lbs. per sq. in. 
47,460 lbs. per sq. in. 
28 per cent. 
54.6 per cent. 



286 STEEL AND ITS HEAT TREATMENT 

Failures of Heat-treated Axles. — Aside from piping, segrega- 
tion, and other impurities in the steel, improper heat treatment is 
the active cause of failure of both heat-treated carbon and alloy 
steel axles. Unequal or insufficient heating in either the hardening 
or toughening processes will produce unequal stresses, which in 
turn will sooner or later result in failures. These failures are 
always transverse, and never longitudinal. Water quenching large 
sections has a strong tendency to produce cracks, often not appear- 
ing on the surface, and which may open up when subjected to the 
heavy duty and " pounding " when placed in service. Such defects 
may be sometimes discovered by the drop test, but its expense 
prevents many railroads from using it for the test of every axle. 
Heat-treated axles, when given ample reduction in the forging 
operation, carefully and uniformly heated to the proper tempera- 
tures, and held at those temperatures for a time sufficient for the 
steel to respond throughout, should prove vastly, superior to un- 
treated or annealed axles. 

On the other hand, the engineering departments of many rail- 
roads have become considerably alarmed over the frequent failures 
of so-called " heat-treated " axles, and many have absolutely refused 
to have anything to do with axles which have been oil or water 
quenched. This really serious phase of the axle question has led 
to the investigation of the possibilities of hardeDing such forgings 
in air or steam. Surprisingly good results have been obtained by 
methods based upon this system — and from the technical stand- 
point are indeed remarkable, since a considerable toughening heat is 
necessary with even 0.40 per cent, carbon steel. 

0.45-0.60 CARBON STEEL 

Treatment of Large Sections. — As the carbon content is pro- 
gressively increased beyond 0.45 per cent., its effect becomes quite 
noticeable in the added brittleness of the steel. This is strongly 
illustrated by the fact that a general study of heat-treatment prac- 
tice will show that there is very little quenching of large sections 
when the carbon content exceeds the 0.50 per cent. mark. The 
dangers to be encountered, both in the treatment itself and by 
possible fracture in service, almost prohibit such treatment of large 
sections. Any increase in static strength which can be obtained by 
quenching and toughening is most certainly acquired with the 
ever-present danger of cracking, or of starting incipient cracks 
For these reasons it is, therefore, apparent that the full heat treat- 



CARBON STEELS 



287 



ment of large sections, even though it may bring out higher physical 
characteristics in the steel — as is shown by the subsequent figures 
obtained from the treatment of a 0.50 per cent, carbon axle — is 
becoming less and less of a factor in steels of these carbons. 

0.50 Per Cent. Carbon Axle 





Forged. 


Quenched in Water 

from 1400° F. 

Toughened at 1200° 

F. 


Tensile strength, lbs. per sq. in 

Elastic limit, sq. in 

Elongation, per cent, in 2 ins 

Reduction of area, per cent 


83,430 

34,370 

22.6 

30.0 


90,090 

53,655 

27.1 

52.8 



Tempering, and Small Sections. — On the other hand, the harden- 
ing and tempering (as distinguished from toughening) of the smaller 
sections, such as gears, dies, etc., begins to take an important place 
in heat-treatment work with these carbons. In such cases the 
increased-carbon content brings about an inherently possible wearing 
hardness which is developed by hardening and tempering. The 
medium and smaller size sections may be satisfactorily hardened 
in water with but a small proportion of the danger which would 
inevitably result from the water quenching (or even oil quenching) 
of larger sections. And, by varying the reheating temperatures, 
the following approximate physical results may be obtained : 

Elastic limit, lbs. per sq. in 50,000 to 110,000 

Elongation, per cent, in 2 ins 20 to 5 

Reduction of area, per cent 50 to 15 

Annealing. — The commrecial annealing of steel of say 0.50 to 
0.60 per cent, carbon will give a variety of results which in them- 
selves have proven a stumbling block for many a heat treater. 
This is largely due to the prominent part and effect of different rates 
of cooling in relation to the size or mass of the steel. To illus- 
trate: 6X6 in. billets of 0.50 to 0.55 carbon which have been heated 
to 1400° F. and furnace cooled will, in general, meet the specifica- 
tions of 

Tensile strength 80,000 

Elastic limit 40,000 

Elongation 22 

Reduction of area 35 



288 STEEL AND ITS HEAT TREATMENT 

On the other hand, smaller sections, annealed in the same manner 
and in the same furnace, will according to their size, give physical 
results varying anywhere between 

Elastic limit 45,000 to 60,000 

Elongation 20 to 15 

Reduction off area 40 to 30 

In other words, the extreme variability in the rate of cooling, 
as dependent upon the size of section and mass of the steel, its rela- 
tion to the size of the furnace, the degree to which the cooling of 
the furnace may be controlled, and numerous other related factors 
make the commercial annealing of these steels an individual problem 
as far as actual physical results are concerned. 

It is therefore always advisable, if specific physical results must 
be obtained by annealing (used in the broad interpretation of the 
term) , to take first a preliminary test of the steel in the condition as 
received. From such results it will then be evident how much the 
steel must be " let down," and the proper reheating temperature 
may be judged from previous experience or by experiment. Al- 
though annealing at a temperature under the critical range will 
not change the general structure of a pearlitic steel, it will relieve 
the strains and stresses, and thereby improve the steel. But fur- 
ther, the previous elaboration, such as rolling or forging, which the 
steel has undergone, will, in a majority of cases in actual practice, 
leave the steel in more or less of a sorbitic state. Under such 
conditions, a reheating — or commercial annealing — will actually 
change the physical results, even though the annealing temperature 
is under the critical range. 

Such commercial annealing or reheating temperatures may vary 
from 900° F. and upwards through the upper critical range. In the 
author's experience there is little or no change in the physical test 
results through the annealing of such steel at temperatures under 
900° F. or thereabouts. But from this temperature upwards 
the sorbitic constituents will gradually coagulate into the pearlite 
and ferrite, with a corresponding lowering of the static strength 
and increase in the ductility. Consequently, by regulating the 
commercial annealing temperature, the physical results may be 
" let down " to the desired limits. 

If it is desired to change entirely the structure of the steel and 
to obtain the finest grain size possible, with maximum ductility, it 



CARBON STEELS 289 

will be necessary to anneal the steel at a temperature slightly in 
excess of the upper critical range, followed by slow cooling. 

The influence of the rate of cooling, as exerted by air cooling, 
is manifested in the peculiar statement that the tensile strength of 
these hard forging steels may be actually raised by annealing (as 
distinguished from quenching). It is a well-known fact that steel 
of such carbon content when cooled in air at a more or less rapid 
rate through the critical range will take on a noticeable degree of 
hardness. This fundamental principle may be applied to great 
advantage in the treatment of axles — with, of course, certain modi- 
fications — and it is even necessary to reheat or toughen in order to 
lower the tensile strength and obtain the proper ratio of static 
strength to ductility. Such a process is now being developed by a 
large manufacturer of axles, and will in all probability have an 
influence upon the heat treatment of axles and other forgings of large 
section. 

CARBON STEELS WITH OVER 0.60 CARBON 

Treatment in General. — The treatment of high-carbon steel 
develops into the two propositions of hardening and annealing. 
Toughening, as referring to high reheating temperatures subse- 
quent to hardening, is but very little used, due to the fact that these 
steels are too brittle for ordinary structural purposes. Similarly, 
tempering is governed entirely by the degree of hardness required 
by the tool and is dependent, not only upon the chemical analysis, 
but in a larger measure upon the result of the hardening operation. 

Hardening. — The precautions to be adopted in hardening may be 
repeated in the following general summary: 

(1) Use the lowest temperature which will give the desired 

results. 

(2) Heat slowly and uniformly. 

(3) The higher the carbon content, the greater is the degree of 

care which must be used, and, in general, the more 
narrow the hardening temperature limits. 
The temperatures to be used in hardening are largely gov- 
erned by the carbon content, and which, in turn, influences the 
position of the critical range. We may sum up these factors as 
follows: 



290 



STEEL AND ITS HEAT TREATMENT 



Carbon Content. Per Cent. 


Critical Range. °F. 


Hardening Temperature. 


°F. 


0.60 


1340-1380 


1400-1460 




0.70 


1340-1375 


1400-1450 




0.80 


1340-1365 


1390-1450 




0.90 


1340-1360 


1375-1450 




1.00 


1340-1360 


1375-1450 




1.10 


1340-1360 


1375-1430 




1.20 


1340-1360 


1375-1430 




1.30 


1340-1360 


1375-1420 




1.40 


1340-1360 


1375-1420 





In giving the above hardening temperatures we have assumed 
that the previous mechanical and heating operations have left the 
free cementite (in hyper-eutectoid steels — greater than 0.9 per cent, 
carbon) well distributed, or emulsified, throughout the steel. This 
will generally be true when the proper finishing temperatures, 
either in rolling or in forging, have been used. In such cases, 
therefore, it will not be necessary to heat to above the Ac. cm range 
in order to emulsify the free cementite, and following with a sub- 
sequent quenching from slightly above the principal critical range. 
If, however, the previous heating operations have left the free 
cementite in the form of spines or network, it will be mandatory to 
use the double-quenching method in order to spheroidalize this 
free cementite and thus obtain the maximum wearing and cutting 
hardness; for details of such procedure the subject matter in Chap- 
ter VII should be studied. 

Annealing. — The general subject of annealing hyper-eutectoid 
steels has been discussed in Chapter VII. a series of physica 
test results of experiments carried out by Fabry 1 upon the anneal- 
ing of steels with carbon contents of 0.58 to 1.36 per cent., the size 
of bar being 1.18 ins. square, and the selected annealing tem- 
peratures being maintained for three hours, are given in the follow- 
ing tables. 

1 Zs. Fabry, " The Variation in the Mechanical Properties and Structures 
of a Few Special Tool Steels Annealed between 600° and 1000° C." Int. Soc. Tes. 
Mat., 1912. 



CARBON STEELS 
0.58 Per Cent. Carbon Steel — Annealed 



291 



Treat- 
ment. 


Tests. 


Hard- 
ness. 


Microscopic. 


Annealed 
Deg. 
Fahr. 


Tensile 
Strength, 
Lbs. per 

sq. In. 


Elastic 
Limit, 

Lbs. 

per 
sq. In. 


Elonga- 
tion, 
per cent 
in 3.15 
Ins. 


Red. of 
Area, 
per cent 


Brlnell 
No. 


Structure. 


Notes. 


1110 


99,540 


— 


15.8 


43.4 


196 


Free ferrlte and 
pearllte. 


Ferrlte reticulated, meshes 
filled with grainy pearl- 
lte. 


1200 


98,420 


45,510 


17.7 


49.0 


183 


Ferrlte begins to change 
Into pearllte. 


1290 


84,200 


39,820 


20.7 


59.2 


174 


Smaller ferrlte crys- 
tals and pearllte. 


Structure essentially dif- 
fering from other speci- 
mens, because the ferrlte 
Is uniformly distributed. 


1380 


93,860 


36,980 18.6 


43.4 


176 


1470 


96,860 


39,820 


19.1 


36.8 


187 


Free ferrlte and 
pearllte. 


Ferrlte forms a network; 


1560 


96,710 


39,820 


17.9 


35.6 


183 


partly lamellar. 


1650 


98,130 


39,820 


18.6 


36.8 


185 




1740 


93,860 


36,980 


16.7 


33.6 


187 


Network of large ferrlte 
crystals filled with pre- 
dominantly grainy pearl- 


1830 


100,700 


39,820 


13.1 


25.2 


196 


lte. 



Critical range Ac commences at 1337°, maximum at 1355°. 



0.81 Per Cent. Carbon Steel — Annealed 



Treat- 
ment. 




Tests. 




Hard- 
ness. 


Microscopic. 


Annealed 
Deg. 
Fahr. 


Tensile 
Strength, 
Lbs. per 

sq. In. 


Elastic 
Limit, 

Lbs. 

per 
sq. in. 


Elonga- 
tion, 

per cent 

In 3.15 

Ins. 


Red. of 

Area, 

per cent 


Brlnell 
No. 




1110 


102,950 


— 


13.1 


37.6 


212 




1200 


106,400 


42,670 


14.0 


35.6 


207 




1290 


99,540 


39,820 


17.8 


43.4 


187 




1380 


100,700 


31,290 


14.6 


29.4 


183 




1470 


102,840 


31,290 


12.5 


14.8 


196 




1560 


105,250 


36,980 


13.1 


23.0 


187 




1650 


100,400 


31,290 


13.1 


19.4 


203 




1740 


98,980 


31,290 


10.2 


14.0 


207 





Critical range Ac commences at 1328°, maximum at 1337°. 



292 STEEL AND ITS HEAT TREATMENT 

0.92 Per Cent. Carbon Steel — Annealed 



Treat- 
ment. 


Tests. 


Hard- 
ness. 


Microscopic. 


Annealed 
Deg. 
Fahr. 


Tensile 
Strength, 
Lbs. per 

sq. In. 


Elastic 
Limit, 

Lbs. 

per 
sq. In. 


Elonga- 
tion, 
per cent 
In 3.15 
Ins. 


Red. of 

Area, 

per cent 


Brlnell 
No. 


Structure. 


Notes. 


1110 


122,900 


— 


12.0 


23.0 


228 


Euctectlc. 


Grainy pearlite with larger 


1200 


120,600 


42,670 


13.0 


25.2 


217 


grains. 


1290 


98,420 


36,980 


11.5 


33.6 


163 


Structure perfectly homo- 
geneous and essentially 


1380 


91,030 


34,130 


17.8 


43.4 


174 


differing from those of 
other specimens. 


1470 


113,500 


31,290 


10.5 


14.8 


212 


Grainy pearlite. 


1560 


112,100 


— 


9.1 


14.0 


207 




1650 


112,500 


31,290 


8.7 


14.8 


216 


Lamellar pearlite. 


1740 


105,800, 31,290 


9.0 


11.6 


214 




1830 


123,450| 36,980 


6.8 


9.2 


228 


Indications of overheated 
structure. 



Critical range Ac begins at 1346°, maximum at 1355 c 



1.11 Per Cent. Carbon Steel — Annealed 



Treat- 
ment. 


Tests. 


Hard- 
ness. 


Microscopic. 


Annealed 
Deg. 
Fahr. 


Tensile 

Strength, 

Lbs. per 

sq. In. 


Elastic 
Limit, 

Lbs. 

per 
sq. In. 


Elonga- 
tion, 
per cent 
In 3.15 
ins. 


Red. of 

Area, 

per cent 


Brlnell 
No. 




1110 


12°, 550 


— 


9.7 


20.8 


248 




1200 


126,300 


51,200 


12.6 


23.0 


235 




1290 


108,650 


54,040 


10.3 


23.0 


185 




1380 


88,180 


39,820 


19.6 


49.0 


170 




1470 


91,590 


36,980 


16.7 


36.8 


178 




1560 


96,700 


25,600 


10.3 


18.6 


196 




1650 


105,100 


29,580 


6.1 


10.0 


207 




1740 


100,700 


25,600 


6.6 


9.2 


202 


i 


1830 


116,050 


36,980 


6.0 


6.8 


228 





Critical range Ac begins at 1337°, maximum at 1346 c 



CARBON STEELS 



293 



1.36 Per Cent. Carbon Steel — Annealed 



Treat- 
ment. 


Tests. 


Hard- 
ness. 


Microscopic. 


Annealed 
Deg. 
Fahr. 


Tensile 
Strength, 
Lbs. per 

sq. In. 


Elastic 
Limit, 

Lbs. 

per 
sq. In. 


Elonga- 
tion, 
per cent 
In 3.15 
Ins. 


Red. of 
Area, 
per cent 


Brlnell 
No. 


Structure. 


Notes. 


1110 


132,550 


— 


6.2 


11.6 


262 


Free cementlte and 
pearllte. 


Cementlte reticulated, 
meshes filled partly with 
lamellar, partly with 
grainy pearllte. 


1200 


T297700 


67,980 


8.5 


14.0 


255 


Free cementlte begins to 
change Into pearllte. 


1290 


123,450 


48,355 


9.6 


16.2 


288 


Fine-grained 
cementite. 


Structure appears uniform. 


1380 


93,300 


45,510 


14.6 


37.6 


192 


As before, grains finer. 


1470 


90,310 


47,500 


17.3 


36.8 


187 


Free cementlte and 
grainy pearllte. 


Cementlte concentrated 


1560 


93,300 


42,670 


13.7 


27.4 


187 


again Into small crystals. 


1650 


102,100 


32,710 


4.5 


5.0 


209 


Free cementlte, 
grainy and lamel- 
lar pearllte. 


Cementlte crystals larger, 
pearllte partly in lamella?. 


1740 


95,580 


28,446 


4.6 


6.8 


196 


Free cementlte and 
and lamellar 
pearllte. 


Structure essentially al- 
tered. Cementlte forms 
a network with large 


1830 


101,830 


— 


2.6 ' 


4.4 


223 


meshes. Steel Is over- 
heated. 



Critical range Ac begins at 1345°, maximum at 1355°. 



CHAPTER XIII 

NICKEL STEELS 

Nickel Steel. — Nickel may well be said to have been the pioneer 
among the common alloys now used in steel manufacture. Origi- 
nally added merely to give increased strength and toughness over 
that obtained in the ordinary rolled structural steel, the development 
and possibilities of heat treatment have greatly enhanced its value, 
so that nickel steel holds a premier position in alloy steel metallurgy. 

The chief difficulties attendant upon its use have been its tend- 
ency to develop a laminated structure and its liability to, seams. 
But when care is used in its manufacture and rolling, and it is not 
made in too large heats or ingots, and when piping and segregation 
are avoided by confining the finished product to that produced from 
the bottom two-thirds of the ingot, an admirable product for many 
purposes is obtained. 

Nickel steel has remarkably good mechanical qualities when 
subjected to suitable heat treatment and is an excellent steel for case 
hardening. In machining qualities it usually takes first place 
among the alloy steels. 

Strength and Ductility. — Nickel primarily influences the strength 
and ductility of steel in that the nickel is dissolved directly in the 
iron or ferrite, in contradistinction to such elements as chrome and 
manganese which unite with, and emphasize the characteristics of 
the cementitic component. Thus, for the forging grades of ordinary 
nickel steel in the natural condition, the addition of each 1 per cent, 
of nickel up to about 5 per cent, will cause an approximate increase 
of 4000 to 6000 lbs. per square inch in the tensile strength and 
elastic limit over that of the corresponding straight carbon steel, 
without any decrease in the ductility. This influence of nickel 
upon the static strength of steel also increases to some degree with 
the percentage of carbon. To illustrate the effect of nickel upon 
steel in the natural condition: a steel with 0.25 per cent, of carbon 
and 3.5 per cent, nickel will have a tensile strength equivalent to 
that of a straight carbon steel with 0.45 per cent, carbon, a propor- 

294 



NICKEL STEELS 295 

tionately greater elastic limit, and the advantageous ductility of the 
lower carbon grade. 

Necessity for Heat Treatment. — On the other hand, and in con- 
nection with the use of alloy steels in general, it should be borne in 
mind that such steels should be used in the heat-treated condition 
only — that is, not in either an annealed or natural condition. In 
the latter conditions there is a benefit, as compared with straight car- 
bon steels and as illustrated above, but often is not commensurate 
with the increased cost. In the heat-treated condition, however, 
there is a very marked improvement in physical characteristics. 
And closely associated with this is the finely divided state of both 
ferrite and pearlite which characterizes heat-treated nickel steel. 

Nickel vs. the Critical Ranges. — One of the most interesting 
phenomena connected with nickel steel is the effect of nickel upon 
the position of the critical ranges. Nickel, like carbon, has the 
property of lowering the points of the allotropic transformations of 
iron, but in a more marked degree. Just as we have seen, in the 
chapter on Hardening, how " rapid cooling " and " carbon " are 
" obstructing agents " in preventing the transformation of the 
austenite into martensite into pearlite, so likewise does nickel act 
as an obstructing agent. The effect of nickel is obtained through 
a lowering of the Ar ranges, so that the temperatures of the critical 
ranges on cooling may even be brought below atmospheric temper- 
atures. Thus we may have a steel which, without quenching, may 
be pearlitic, troostitic, martensitic or austenitic, dependent upon 
the relative percentages of nickel and carbon. Hence, such steels 
containing nickel may be classified according to their microscopic 
constituents which are obtained upon slow cooling from a high 
temperature. 

Classification of Nickel Steels. — In Fig. 197 there is shown 
graphically the influence of the nickel-carbon ratio upon the struc- 
ture of nickel steels as cast, or as moderately cooled from a high 
temperature. 

The " Pearlitic " nickel steels are those in which the critical 
ranges are all above the ordinary temperatures, so that such steels 
as slow cooled from a high temperature will consist of pearlite 
plus ferrite (or cementite). These are the ordinary commercial 
nickel steels, and are represented by the lower triangle of Fig. 197. 

" Martensitic " nickel steels contain that percentage of nickel 
and carbon which will so lower the position of the critical ranges 
on cooling that only the partial transformation may proceed. That 



296 



STEEL AND ITS HEAT TREATMENT 



is, the austenite is transformed into martensite, but no further — 
the steel being too rigid to allow a more complete transformation at 
the low temperatures involved. These steels correspond to the mid- 
dle triangle in Fig. 197. Nickel steels martensitic throughout 
have no practical value, as it is impossible to work or machine them. 
On the other hand, great importance is attached to the use of cer- 
tain pearlitic nickel steels which can become martensitic upon case 
carburizing — due to the increased carbon content. 



30 



20 



10 



SfJSTi. 







Austenitic 






Martensitic 






Pearlitic 






~-^», 



2C. 



0.40 



0.80 



1.20 



1.60 



Fig. 197.— Influence of the Nickel-carbon Content upon the Structure of Nickel 

Steels as Cast. 



A still further increase in the nickel or carbon content will cause 
the critical range on cooling to fall below atmospheric temper- 
atures, so that such steels will be characterized by an " austenitic " 
or " polyhedral " structure, and are known under these names. 

Micrographic Structure. — These changes in structure are illus- 
trated by the series of photomicrographs (by Savoia) given in Figs. 
198 to 205, all the steels being in the natural condition, having 
approximately the same carbon content (0.25 per cent.), but with 
increasing percentages of nickel. 



NICKEL STEELS 



297 




Fig. 198.— Steel with 0.25 per cent. Carbon, 2 per cent. Nickel. X650. 

(Savoia.) 







Fig. 199. — Carbon, 0.25 per cent. Nickel, 5 per cent. X650. 
(Savoia.) 



298 



STEEL AND ITS HEAT TREATMENT 




Fig. 200.— Carbon, 0.25 per cent. Nickel, 7 per cent. X650. 
(Savoia.) 




Fig. 201.— Carbon, 0.25 per cent. Nickel, 10 per cent. X650. 
(Savoia.) 



NICKEL STEELS 



299 




Fig. 202.— Carbon, 0.25 per cent. Nickel, 12 per cent. X650. 
(Savoia.) 



; 




Fig. 203. — Carbon, 0.25 per cent. Nickel, 15 per cent. X650. 
(Savoia.) 



300 



STEEL AND ITS HEAT TREATMENT 



+-«7. <«Rf*« W •.■S4& :.:& 



* ill ••* 



Fig. 204.— Carbon, 0.25 per cent. Nickel, 20 per cent. X650. 
(Savoia.) 




Fig. 205.— Carbon, 0.25 per cent. Nickel, 25 per cent. X650. 

(Savoia.) 



NICKEL STEELS 



301 



It will be seen that the structure of the 2 per cent, nickel steel 
(Fig. 198) is similar to that of a corresponding straight carbon steel, 
but is finer and more homogeneous. 

The 5 per cent, nickel steel (Fig. 199) shows a still finer and 
denser structure, in that the pearlite is more divided and distributed. 

With the 7 per cent, nickel (Fig. 200) the ferrite and pearlite 
are still seen, but they are distributed in a special manner as if 
disturbed by the approach of a transformation. A tendency to 
orientiate, somewhat like martensite, is also noticeable. 



Peaiiitic 



Martensitic 



Austenitic 




200,000 



150.000 



100,000 



#Ni 



Fig. 206. — Comparative Physical Properties of Nickel Steels with 0.25 per cent. 

Carbon. 



The steels with 10 and 12 per cent, nickel (Figs. 201 and 202) are 
both wholly martensitic. 

With the 15 per cent, nickel (Fig. 203) intensely white con- 
stituents appear amidst the martensite and probably represent the 
first appearance of austenite. The latter increases quite noticeably 
in the steel with 20 per cent, nickel (Fig. 204), taking on its poly- 
hedral form. 

At 25 per cent, nickel (Fig. 205) the whole steel is characterized 
by large polyhedra of gamma-iron. 

Physical Properties with Increasing Nickel. — The physical 
properties of these same cast nickel steels are plotted graphically in 



302 



STEEL AND ITS HEAT TREATMENT 



the chart in Fig. 206. It will be noted that the abrupt changes in 
the curves correspond very closely with the theoretic structure given 
by the upper abscissae ; and that these same physical properties are 
indicative of the essential characteristics of pearlite, martensite and 



°F. 

1600 


\ 
\ 
\ 
\ 
\ 
\ 










1500 


\ 
\ 
\ 

\ 

\ 

\ 

\ 
\ 


\ 

\ 










\ 

\ 
\ 








1400 












"\^ 


">-/ _ . 




u o 




1300 










nr 








o->-^/ 




1200 




1 















jiC. 0.30 0.40 0.60 0.80 

Fig. 207. — Critical Changes on Heating 3 per cent. Nickel Steel. 



1.0 



austenite, as denoted by the tensile strength, ductility (elongation) 
and resistance to shock. 

Critical Range of Pearlitic Steels.— For nickel steels with less 
than 5 to 7 per cent, nickel, each 1 per cent, nickel lowers the crit- 
ical range (Acl) on heating by about 15° to 20° F., and also lowers 
the Arl range (cooling) by about 30° to 40° F., below those of the 



NICKEL STEELS 



303 



corresponding ranges for straight carbon steels of the same carbon 
and manganese contents. Similarly, there is a corresponding lower- 
ing of the other critical ranges. 

This is graphically shown in Fig. 207, which has been carefully 
plotted from a series of observations obtained with 3 per cent, 
nickel steels of various carbon contents. It will be seen from this 
curve that the critical ranges on heating are about 60° F. below the 
corresponding straight carbon steels. With the very low carbons 
there appears to be a tendency for the Ac3 curve to flatten out; this 
is further substantiated by results with steels containing 5 to 7 per 
cent, nickel. Beyond the eutectoid ratio of carbon it was found 
that the Ar range would begin to drop quite rapidly (not shown in the 
diagram) below its normal value, as might be expected from the 
fact that an increase in carbon in these steels act in an analogous 
manner to an increase in nickel. 

The approximate temperatures of the Acl and Ar ranges for 
nickel steels are as follows; 



Per Cent. Nickel. 


Acl, ° F. 


Ar, ° F. 





1340 


1280 


1.0 


1320 


1240 


2.0 


1300 


1200 


2.5 


1290 


1180 


3.0 


1280 


1160 


3.5 


1270 


1140 


4.0 


1260 


1120 


4.5 


1250 


1100 


5.0 


1240 


1080 


6.0 


1220 


1040 


7.0 


1200 


1000 



It must be borne in mind, however, that the Acl temperatures may 
vary considerably from steel to steel — but those given above will 
probably be about the average of those obtained in practice, and 
will in any event be within ±25° F. On the other hand, the Arl 
temperatures given are liable to an even greater variation, as the 
maximum temperature attained in heating, the length of time 
occupied in both heating and cooling, the effect of the higher car- 
bon contents, and many other experimental factors, all tend to 
change the position of the Arl range. 

From these figures, and from the critical range diagram given 
for 3 per cent, nickel steels, it will be observed that the hardening of 



304 STEEL AND ITS HEAT TREATMENT 

nickel steels may be carried out at temperatures considerably lower 
than those required by the corresponding straight carbon steels. 

The Eutectoid for Nickel Steels. — The effect of additional 
nickel, or at least up to 7 per cent., is to reduce the eutectoid carbon 
ratio below that of the 0.9 value for straight carbon steels. That is, 
a nickel steel with 3 per cent, nickel will be saturated, having neither 
excess ferrite nor excess cementite (on slow cooling), at about 0.75 
to 0.8 per cent, carbon; while in 7 per cent, nickel steel the eutec- 
toid ratio appears to be about 0.6 per cent, carbon. This fact is of 
great importance in case-hardening work, in that it not only permits 
of a shorter duration of the carburization in order to obtain the 
maximum carbon concentration necessary in the case, but also 
reduces the carbon content over which it is likely that enfoliation 
may occur. 

Heat Treatment of Pearlitic Nickel Steels.— The heat treatment 
of pearlitic nickel steels presents some very interesting phenomena 
which are quite distinctive from ordinary straight carbon steels. 
One would naturally assume that the treatment of pearlitic nickel 
steels would be carried out in an analogous manner to that of the 
ordinary carbon steels — that is, the quenching should be done at a 
temperature slightly in excess of the upper critical range, provided 
that the duration of heating at the maximum temperature has been 
sufficient to effect the entire solution of the previous components, 
together with their diffusion and the equalization of the steel as a 
whole. Similarly, as in carbon steels, we would assume that we 
might replace the length of heating at the proper quenching temper- 
ature by a higher temperature in order more quickly to effect the 
equalization of the steel; provided, however, that this new and 
more elevated temperature shall not produce too great a deteriora- 
tion in the metal through increase in grain-size, etc. — or that this 
higher quenching is followed by a quenching at the proper tempera- 
ture. In straight carbon steels the change of structure by heating 
slightly above the upper critical range takes place quickly as a general 
rule; and the coarsening or embrittling of the steel also occurs 
rapidly when higher temperatures are used. The influence of 
nickel in the steel, however, often necessitates a modification, or 
permits a simplification, of these general principles, both in regard 
to the temperature of quenching and the length of heating. 

In the first place, the addition of nickel appears to make the 
solution of the ferrite or cementite and the equalization of the steel 
as a whole take place more slowly than in the ordinary carbon 



NICKEL STEELS 305 

steels. Thus, if we take a steel containing some 4 or 5 per cent, 
nickel, and a mild or medium carbon content, and quench it 
after a normal heating at a temperature some 50° F. over the 
critical range, the transformation is often incomplete and the 
martensite not uniformly distributed nor equalized. 

In such an event, which is usually characteristic of nickel steel 
which has either undergone a more or less severe elaboration or work- 
ing, or has been finished at too low a temperature, or has been sub- 
jected to a prolonged heating at some high temperature, there are 
then four methods cf procedure available : 

(1) Prolonged heating at the proper quenching temperature 

to effect the necessary transformation, followed by 
quenching; 

(2) Heating to a higher temperature than in (1), and quench- 

ing; 

(3) Heating to the higher temperature, cooling to a temper- 

perature a little above the Ar temperature, and then 
quenching; 

(4) Quenching, or air cooling, from the higher temperature, 

followed by a normal reheating to a temperature slightly 
in excess of the Ac range, and quenching. 

These propositions at once evoke a discussion of further char- 
acteristics which the presence of nickel involves. 

If an ordinary carbon steel be heated for a considerable duration 
of time at a temperature even slightly over the critical range, the 
grain-size will begin to increase, with a corresponding decrease in 
both the static and dynamic strength of the material. On the 
other hand, if a nickel steel be subjected to a length and temperature 
of heating equivalent to that of the carbon steel, the pearlite and 
ferrite grains will remain (after slow cooling) considerably finer, 
more uniformly distributed, and much more subdivided than the 
carbon steel. This characteristic permits the greater duration of 
heating as required under the first proposition, without any per- 
ceptible deterioration such as would be noticeable in a straight car- 
bon steel with the same prolonged heating. However, such treat- 
ment — when required — is disadvantageous from the commercial 
standpoint, as it decreases the capacity of the heat treatment plant, 
with a corresponding increase in the cost of production. 

Again, the increased brittleness due to a more or less prolonged 
heating at temperatures in excess of the upper critical range is con- 
siderably less for nickel steels than for ordinary carbon steels. This 



306 STEEL AND ITS HEAT TREATMENT 

fact is well illustrated by the following results upon a straight carbon 
steel in comparison with a 2 per cent, nickel steel of the same carbon 
content, taken from a memoire 1 by Guillet : 



Length of Heating 


Resistance to Shock. 


at 1830° F. 


Ordinary extra-soft steel. 


Extra-soft steel with 
2 per cent nickel. 


Normal heating 


20 kgms. 
4.5 kgms. 
4.0 kgms. 


60 kgms. (not broken) 


Four hours 


60 kgms. (not broken) 


Six hours 


60 kgms. (not broken) 





If, in order to obtain the full equalization of the steel and also to 
avoid a prolonged heating at the lower and theoretic temperature, 
it shall be necessary to heat and quench from a higher temperature, 
such operation may be undertaken without that fear of greatly 
increasing the brittleness which would most probably occur in a 
straight carbon steel. 

Although it is granted that a heating to this high temperature 
may be necessary, a quenching from this same high temperature 
would not be entirely logical if this were to be the only quenching, 
and also if viewed from the standpoint of the best product. In 
such high temperature quenchings there is the ever-present danger 
of cracking and warping. Further, it is a generally admitted fact 
that no change in the molecular arrangement of the steel occurs 
in cooling such a steel until the upper critical range on cooling (Ar3) 
is reached. Assuming this to be true, we may then mod'fy the previ- 
ous treatment (proposition 2) by first cooling the steel — after heat- 
ing to the high temperature — to a temperature slightly above that 
of the Ar3 range, and then quench, as stated under proposition 3. 
This treatment will retain all the benefits which may accrue from 
the original high-temperature heating, but at the same time will 
diminish to a considerable degree the dangers of cracking and 
warping. And as the critical ranges on cooling in nickel steels are 
even further below those of the Ac ranges in comparison with 
straight carbon steels, this quenching temperature will be reason- 
ably low. __ 

Objections which may be offered to this method are that the 
quenching from just over the Ar range may not give the maximum 

X M. L. Guillet, " Traitements thermiques des aciers speciaux," Rev. de 
Met., July, 1910. 



NICKEL STEELS 307 

hardening effect unless the quenching temperature has been gauged 
just rightly, or if the carbon content is low. The first objection may- 
be overcome by suitable temperature control; if the quenching 
temperature should fall too low, the difference in the hardening 
effect, for forgings or full-heat treated work, may be later corrected 
by using a lower toughening or drawing temperature. By the use 
of exact methods, such as one furnace maintained at the high tem- 
perature, and then another furnace (into which the steel may be 
subsequently placed) maintained at the temperature a little over the 
Ar range, the first objection may be entirely cleared away. The 
second objection may also be at once overruled by the fact that 
the treatment of the low-carbon steels is generally limited in com- 
mercial work to the obtaining of a suitable toughness and absence 
of brittleness (regeneration), and that it is usually not desired to 
obtain the maximum hardness. 

In brief, it does not matter whether the same mechanical prop- 
erties in a pull test are obtained by a quenching made at a very 
high temperature, or by a quenching at a lower temperature follow- 
ing the return. As these results in the mechanical properties are 
practically the same, the treatment under proposition 3, as compared 
with Number 2, is always more advantageous from the point of 
view of non-brittleness and probably also from the point of view of 
the strength of the piece. 

The most serious objection to the treatment in either (2) or (3), 
however, is the increase in brittleness which is liable to occur if 
the high temperature heating is unduly prolonged. Although the 
presence of nickel tends to diminish such a condition, the effect 
of high heating is always towards the increase in grain-size, and 
coarse martensite generally corresponds to a diminution in the 
strength of the steel. Assuming that a temperature considerably 
in excess of the upper critical range is mandatory, any ill effects 
resulting therefrom may be entirely overcome by a double heating 
and cooling, and yet also retaining the benefits of such high temper- 
ature heating. That is, by cooling the steel — but not quenching, 
unless the original structure is very bad indeed; or unless the most 
perfect structure is desired — from the high temperature to a tem- 
perature under that of the Al range, in order to impress the effect 
of the high temperature upon the steel, followed by a reheating 
to a temperature slightly in excess of the upper critical range, and 
then quenching. Such a hardening treatment, either with air cool- 
ing and a subsequent single quenching, or with a double quenching, 



3081 STEEL AND ITS HEAT TREATMENT 

is the best, although the most expensive. As this method has been 
discussed in its relation to carbon steels, and as its influence is 
approximately the same with pearlitic nickel steels, it will not be 
necessary to dwell further upon it. 

In general, the treatments (for the best quenching effect) given 
under (1), (2) and (3) will suffice for ordinary commercial practice, 
but with the preference given to (1) or (3). That under (4) is best 
if the higher cost is permissible. 

Moreover, it should be borne in mind that, in perhaps even a 
majority of cases, the regular and normal quenching from a tem- 
perature slightly in excess of the upper critical range (Ac3), after 
a thorough and uniform heating at that temperature, will generally 
suffice — and especially for small work. But in order more fully to 
explain the difficulties which are sometimes met with in the treat- 
ment of nickel steels, the author has entered into the foregoing 
explanations. As a safe and general fundamental principle, re- 
peatedly urged, it is. always advisable to quench from the lowest 
temperature which will give the desired results. 

The tempering and toughening of pearlitic nickel steels is carried 
out exactly as with straight carbon steels, previously explained, and 
is dealt with in more detail later on in the chapter. 

CARBURIZATION OF NICKEL STEELS 

The general principles of the carburization of nickel steels are 
similar to those which apply to straight carbon steels, and should 
not require repetition. There are, however, certain peculiarities, 
presenting both advantages and disadvantages, which should be 
mentioned. 

(1) Nickel steels show less susceptibility to brittleness due to 
prolonged heating at the high temperatures often used in carburiza- 
tion than do the corresponding carbon steels. This important fact 
not only gives a steel better able to withstand shock, but also gives 
a well-defined means of simplifying the subsequent heat treatment 
if desired. Such advantages may be readily obtained by the addi- 
tion of even 2 per cent, of nickel, and largely compensate for the 
slightly higher cost. 

(2) The variations in the concentration of the carbon in the 
carburized zones are more gradual and uniform in nickel steels than 
in carbon steels. This better distribution of the carbon therefore 
tends towards the prevention of a distinct line of demarkation be- 
tween the different zones, and thus to eliminate the chipping and 



NICKEL STEELS 



309 



flaking off of the case. Similarly, the phenomenon of " liquation," — 
a principal factor in such enfoliation — is less marked, under equal 
conditions, in nickel steels than in carbon steels. 

(3) Although it is true that carburization proceeds with greater 
slowness with some solid carburizing compounds, referring to their 
use with nickel steels with less than say 3.5 per cent, nickel, the use 
of a mixed cement (carbon monoxide plus carbon) will effect a car- 
burization with a rapidity equal to that with ordinary carbon steels. 

(4) Under the same conditions, the depth of penetration of the 
carburized zone for a given time, using a mixed cement, is even 
slightly higher for nickel steels than for carbon steels. 

(5) The lesser hardness which, with the same treatment, is 
possessed by the carburized zones in nickel steel as compared with 
the carburized zones in carbon steels under identical conditions, is 
due not only to the different effects of different quenchings, but 
also to the smaller concentration (especially for less than 3 per cent, 
nickel) of carbon in the carburized zones. This disadvantage may 
be eliminated by raising the carbon in the carburized zone by a 
suitable change in the conduct of the carburization. 

(6) When the nickel content exceeds 3 per cent, the maximum 
concentration of the carbon in the carburized zones decreases with 
an increase in the percentage of nickel contained in the steel. The 
following table, from Giolitti, 1 contains data relative to the maximum 
concentration reached by the carbon in the carburized zones when 
carburizing, under various conditions, steels with varying nickel 
contents : 



Condition of Carburization. 


Nickel Content. 




2% 


3% 


5% 


25% 


30% 


Carbon monoxide: 

5 hours at 1740° F 




0.38 
0.35 

1.53 


0.23 
0.35 

0.93 

1.28 

0.70 
0.80 
0.73 
0.74 
0.83 


0.84 

0.64 
0.59 

0.73 


15 


5 hours at 1920° F 




17 


Ethylene : 

5 hours at 1740° F 


1.12 


39 


5 hours at 1920° F 


63 


Mixed cement: 

2 hours at 1830° F 


0.70 
1.12 
0.83 
0.92 
1.07 


67 


5 hours at 1830° F 




5 hours at 1920° F 




2 hours at 2010° F 


40 


5 hours at 2010° F. . . 









X F. Giolitti, " The Cementation of Iron and Steel." 



310 



STEEL AND ITS HEAT TREATMENT 



(7) By employing a nickel steel of the proper nickel content, and 
carburizing in such a manner as to attain a definite maximum 
carbon concentration in the case, a steel characterized by a tough 
core and a martensitic structure in the case may be obtained with- 
out subsequent quenching. The approximate maximum carbon con- 
centration in the case which it is necessary to obtain for steels with 
definite percentages of nickel in order to produce a martensitic struc- 
ture without quenching, may be given about as follows: 



Per cent. Nickel. 


Per cent. Carbon. 


Per cent. Nickel. 


Per cent. Carbon. 


2 
3 
4 


1.50 
1.30 
1.10 


5 
6 

7 


0.95 
0.85 
0.75 



Such methods eliminate the necessity for subsequent heat treat- 
ment, if so desired, and effect corresponding reductions in the cost 
of the process, besides obviating, in a large measure, such important 
factors as warping, grinding, etc. Further, by extending the car- 
burization so as to reach a maximum of 1.5 per cent, carbon at the 
periphery, for steels containing 5 to 7 per cent, nickel, there can 
also be obtained a superficial layer, superimposed upon the mar- 
tensitic zone, containing austenite, which easily admits of polishing 
without loss. 

(8) The lower temperatures at which the critical ranges are 
located, in the pearlitic nickel steels, permit a lower temperature 
to be used in case carburizing, which is an important factor in 
intricate or exact work. 



THEKMAL TREATMENT AFTER CARBURIZATION 

In general, the thermal treatment of nickel steels, subsequent 
to case carburizing, may be classified according to the structure of 
the case after slow cooling from the temperature of carburization — 
that is, whether it is pearlitic or martensitic. Although this struc- 
ture depends primarily upon the conduct of the carburization and 
the maximum carbon concentration thus obtained in the case, the 
procedure as practically carried out in commercial work will usually 
give (upon slow cooling after carburization) (1) a pearlitic structure 
in the case for steels with nickel contents under 4 per cent, and (2) a 
more or less martensitic case for steels with 4 to 7 per cent, nickel. 



NICKEL STEELS 



311 



As explained in Chapter XI the best treatment which can be 
given any case-carburized pearlitic steel is that involving a double 
quenching. Each quenching — for regeneration and for hardening — 
should be carried out at the most suitable temperature, and which is 
fixed by the transformation points of the core and case respectively. 
These treatments for nickel steels with 2 to 2| and 3 to 3| per cent, 
nickel are approximately as follows: 

Carburization. — Carburize at the desired temperature, usually 
1600° to 1750° F. Cool slowly in the carburizing material 
(assuming solid cements). 

Thermal Treatment. — 



Nickel Content, Per cent. 


2 to 2\. 


3 to 3|. 


Carbon Content, Percent. 


0.10 to 0.15 


0.15 to 0.20 


0.10 to 0.15 


0.15 to 0.20 


Regenerative 

quenching 

Hardening 

quenching 


(a) 1550-1600° 
or (6)1600° 

1325-1375° 


1500-1550° 
1600° 

1325-1375° 


1475-1525° 
1600° 

1300-1350° 


1450-1500° 
1600° 

1300-1350° 



The steel may be removed from the quenching bath as soon as 
it loses its red color during the regenerative quenching, and imme- 
diately reheated for the second quenching. Practice differs as to 
the temperature to be used for the regenerative quenching, some 
preferring to quench from a temperature slightly above the Ac3 
range, as under (a), while others prefer to use the higher temper- 
ature (b). The reasons for these have been discussed in previous 
sections. 

On the other hand, for 2 per cent, nickel steels, Guillet recom- 
mends temperatures distinctly higher than those given above — 
which probably coincide with the best American practice — for the 
regenerative quenching, and which he gives as follows: 

Regenerative quenching 1760° to 1800° 

Hardening quenching 1365° to 1420° 



The hardening quenching should be conducted at the lowest pos- 
sible temperature at which the metal of the case will become glass- 
hard. In many instances it will be found that temperatures some- 
what lower than those given in the above table can be used. For 



312 



STEEL AND ITS HEAT TREATMENT 



example, the critical curve given in Fig. 208 for a steel with 0.13 per 
cent, carbon, 0.49 per cent, manganese and 3.35 per cent, nickel, 
shows the Acl range to be about 1250° F., so that a temperature 
under 1300° to 1350° might be used effectively for the hardening 
quenching. 

The effect of different treatments upon Guillet's 2 per cent, 
nickel steel in its resistance to shock is shown in the following table : 

Treatment. Resistance to Shock. 

Steel with 2 per cent, nickel and 0.1 per cent, carbon: 

Heated to 1700° F. and air cooled 33 . 4 kgms. 

Quenched in water from 1700° F 34.5 

Same steel cemented at 1830° F. for 1.2 mm.: 

Slow cooled 31.0 

Quenched in water from 1830° F 33 . 5 

Double-quenched in water, 1830° and 1380° 36.0 

Quenched in water from 1380° F 32.0 




Fig. 208. — Critical Range Diagram. 



Simplified Thermal Treatments after Carburization. — On account 
of the fact that the brittleness of the core (with nickel steels) is not 
greatly increased by the heating during carburization if the tempera- 
ture of that operation is not too high, and as the Ac3 range of the 
ordinary nickel steels is considerably lower than that of the corre- 
sponding straight carbon steel, it makes it possible, as we have seen, 
to effect a regenerative quenching at a temperature in the neighbor- 
hood of 1500° -1550° F. Further, as the nickel steel case can be 



NICKEL STEELS 



313 



hardened at a temperature considerably above the normal Acl 
without losing too much of its hardness or increasing too largely in 
brittleness, it follows that, in many instances, the regenerative 
quenching may also serve as a hardening quenching. This permits 
the simplification of the treatment to a single quenching for nickel 
steels, unless the piece is to be subjected to exceptional stress. The 
practical usefulness of this method is obvious. 

It is evident, however, that the double quenching will always 
give considerably better results for both core and case. This is 
particularly shown in the depth and degree of hardness obtained by 
the lower quenching over the higher quenching temperature by the 
following experiments by Guillet on 2 per cent, nickel steels: 



Treatment. 


Shore Hardness Numbers. 


Maximum. 


Minimum. 


Mean. 


Cemented pieces, not quenched 

Cemented pieces, quenched from 1830° F. 
Cemented pieces, quenched from 1380° F. 


40 

84 
88 


39 
69 
85 


39.37 
78.05 
86.56 



Case Hardening by Air Coolings — Again, the case-hardening 
process may be even further simplified by the use of nickel steels with 
3.5 per cent, of nickel, or more, and conducting the carburization in 
such a manner as will produce a maximum carbon content in the case, 
which will give a martensitic structure on air cooling from the tem- 
perature of carburization. An example of this is shown in Figs. 209 
and 210 representing a case-carburized steel with an initial carbon 
content of 0.176 per cent., with 3.44 per cent, nickel; the steel was 
then air cooled directly after carburization. The thickness of the 
martensitic zone is about 0.5 mm. Under the lower magnification 
(Fig. 210) a solid troostitic band is seen to separate the martensite 
and the sorbito-pearlite portions. The principal advantage which 
this method presents consists of its great simplicity, and also in the 
fact that it permits the avoidance of deformation which so often 
accompanies any quenching operation. Nickel steels which are 
martensitic after air cooling may be troostitic, sorbitic, or even pearl- 
itic after very slow cooling in the furnace, while they may be austen- 
itic on water quenching. 

Case Hardening 5 to 7 Per Cent. Nickel Steels. — Advancing 
another step and using nickel steels with 5 to 7 per cent, nickel, we 
find that the ordinary carburization and subsequent slow cooling 



314 



STEEL AND ITS HEAT TREATMENT 




Fig. 209.— Nickel Steel. Nickel, 3.44 per cent. Carbon, 0.176 per cent. 
Case Carburized and Air Cooled. X 100. (Sauveur and Reinhardt.) 




Fig. 210. — Same Steel and Treatment as in Fig. 162. 
(Sauveur and Reinhardt.) 



X50. 



NICKEL STEELS 



315 



will produce a case with characteristics varying over a wide range, 
dependent upon the nickel-carbon ratio in the case. In other words, 
the transformation range of the metal of the case on cooling may be 
even further reduced below that of the previous example, giving 
either a martensitic or martenso-austenitic structure upon slow cool- 
ing. Therefore, when it is not required to produce an extremely 
tough core, nor to obtain extreme hardness in the case, the carbur- 
ized pieces with 5 to 7 per cent, nickel may simply be allowed to 
cool slowly in the carburizing mixture after carburization. 

The use of the method just indicated, however, has its dis- 
advantages. The following table shows the results of scleroscope 
hardness tests made by Guillet on a steel containing 7 per cent, 
nickel and 0.12 per cent, carbon, carburized to a depth of 0.1 mm., 
but not quenched: 



Treatment and Tests. 


. Shore Hardness Numbers. 


Mean. 


Maximum. 


Minimum. 


Test made on the surface 


18.5 
26.5 
24.5 
20.2 


21 

27 
25 
20 


17 


Test made at a depth of 0.2 mm 

Test made at a depth of 0.4 mm 

Test made at a depth of 0.6 mm 


26 
24 
21 



From this particular instance it will be seen that the surface zone 
is partly austenitic, so that a very great hardness is not obtained. 

In the second place, it is evident that the core of the piece thus 
treated has not been regenerated, although, as we have said before, 
nickel steel does not have the maximum brittleness which a straight 
carbon steel would have under the same conditions of cooling. 

The structure of a steel containing 4.86 per cent, nickel and 0.115 
per cent, carbon, intensely carburized, and air cooled, is shown in 
Fig. 211. This photomicrograph shows that the effect of such 
treatment is to produce a case which is largely austenitic. 

The best practice, however, both American and foreign, specifies 
the use of a double-quench treatment subsequent to a mild carburiza- 
tion, and using a soft steel with 4.5 to 6 per cent, nickel. Such steels 
have many peculiar advantages: the carburization may be con- 
ducted at a moderate temperature; a maximum carbon content in 
the carburized zone of only about 0.45 to 0.6 per cent, is necessary 
to produce a glass-hard surface on oil quenching; and the core 
becomes exceedingly strong, as well as tough and non-brittle. From 



316 



STEEL AND ITS HEAT TREATMENT 



these facts it is evident that the lowering of the maximum carbon 
concentration to a percentage not exceeding that of the. eutectoid 
ratio will almost entirely eliminate the danger of chipping and 
flaking. The use of moderate temperatures for carburization and 
of oil for quenching decreases the liability to warping and fracture. 
The physical characteristics of the carburized zone after the second 
oil quenching, the steel of the case having an approximate chemical 




Fig. 211. — Nickel Steel. Nickel, 4.86 per cent. Carbon, 0.115 per cent. 
Case Carburized and Air Cooled, X100, (Sauveur and Reinhardt.) 



composition of 0.45 per cent, carbon and 5.0 per cent, nickel, will 
be approximately: 

Tensile strength, lbs. per sq. in 260,000 

Elastic limit, lbs. per sq. in 250,000 

Elongation in 2 ins., per cent 2 

Reduction of area, per cent 5 

Brinell hardness 490 

Scleroscope hardness 74 



NICKEL STEELS 317 

The following physical results taken from the core of a double- 
quenched steel analyzing: 

Carbon 0.105 

Manganese . 43 

Phosphorus . 014 

Sulphur 0.030 

Silicon 0.11 

Nickel 5.0 

show that the core will have great strength, high ductility (from the 
reduction of area) , and very slight brittleness (as shown by the shock 
test) : 

Tensile strength, lbs. per sq. in 200,000 

Elastic limit, lbs. per sq. in 170,000 

Elongation in 2 ins., per cent 12 

Reduction of area, per cent 54 

Resistance to shock 75 

Brinell hardness 295 

The same steel, having an upper critical range of 1425° F., and 
annealed at 1475° F., gave: 

Tensile strength, pounds per square inch. . . . 90,600 

Elastic limit, pounds per square inch 60,160 

Elongation in 2 ins., per cent 20 

Reduction of area per cut 60. 5 

Resistance to shock 116 

Brinell hardness 179 

The regenerative quenching for these steels may be carried out 
either at a temperature slightly in excess of the upper critical range — 
or at about 1475° F., or, in order to more fully equalize and effect 
the regeneration of the core, at some higher temperature, such as 
1600° F. The hardening quenching temperature should be slightly 
over the Ac range of the case, or approximately 1275° to 1325° F. 
Oil may be used for both quenchings. For the characteristic French' 
steel containing about 6 per cent, nickel Gurnet recommends the 
temperatures of 1560° and 1250° F. respectively for the doublej 
quenching. 

3.5 PER CENT. NICKEL STEEL 

We have previously discussed some of the factors which enter 
into the quenching of nickel steels in general. Whether or not it 



318 



STEEL AND ITS HEAT TREATMENT 





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NICKEL STEELS 



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NICKEL STEELS 



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fH 



NICKEL STEELS 



323 



may be necessary to use a temperature considerably in excess of the 
upper critical range for hardening will depend upon the condition of 
the steel as it comes to the heat-treatment plant; in many cases the 
normal heating and quenching will suffice for general purposes. 
The normal hardening temperatures for 3 to 3^ per cent, nickel steel 
may be approximately determined by reference to the critical range 
diagram in Fig, 207, and by adding 50 to 100 degrees to the upper 



coo 



a 400 



































Effect of Mass 
C. 0.25 P. 
Si. 0.09 S. C 


01 
















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Size in Inches 



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Fig. 217.— Effect of Mass upon the Hardness of Nickel Steel, Oil Treated. 
(Matthews & Stagg,) 



critical range for the given carbon content. It should be noted, 
however, that the best hardening temperature should be determined 
experimentally, whenever possible, for the particular stock to be 
treated, since the method of manufacture, elaboration, size of sec- 
tion, and various other factors all influence such temperature. 

The normal characteristics -obtained by the heat treatment of 1-in. 
nickel steel rounds are shown in the charts of Figs. 212 to 216. It 



324 



STEEL AND ITS HEAT TREATMENT 



should be remembered that these figures, although they have been 
carefully checked up with other results as far as is possible, are 
experimental figures, and should be used with discretion. In other 
words, ordinary commercial heat-treatment practice involves so many 
variables, and especially the " personal equation," that it should 
not be expected that these results will be duplicated in every instance. 



































Effect of Mass 
C. 0.20 P. 0.01 
Si. U.09 S. 0.012 
Mn. 0.07 Ni. 3.4? 














































































































































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Size in Inches 



2^ 



Fig. 218. — Effect of Mass upon the Hardness of Nickel Steel, Water Quenched. 

(Matthews & Stagg.) 



Again, these results represent the treatment of 1-in. round sections — 
so that while such results might be duplicated in' practice with 
sections up to If ins. in diameter, further increase in the size of 
section will inevitably lower the physical test results to be obtained 
under the same treatment. The influence of mass upon the Brinell 
hardness is shown in Figs. 217 and 218. 

On the other hand, the normal characteristics for annealed 3| per 
cent, nickel steel, as given in the diagram in Fig. 219, represent the 



NICKEL STEELS 



325 



average results which are, and should be, obtained in commercial 
practice in the annealing of the more common and larger sections of 
nickel steel. They represent, moreover, the minimum requirements 
which are characteristic of many existing steel specifications for 
3§ per cent, nickel steel, annealed, for such uses as engine forgings, 



100,000 



& 90,000 




H 40,000 



30,000, 



0.20 



0.25 



0.30 



0.35 
Per Cent. Carbon 



0.40 



0.45 



0.50 



Fig. 219. — Normal Characteristics of Annealed 3.5 per cent. Nickel Steel. 
Large-size Sections of Forgings. Manganese Approx. 0.6 per cent. 



ordnance forgings, rolled slabs and billets, etc., both for Govern- 
ment and commercial uses. 

Similarly, the following physical results for heat-treated work 
(quenched and toughened) have been taken from various specifica- 
tions in order to show the minimum results which may be expected 
in commercial practice. The manganese requirements are approx- 
imately 0.50 to 0.70 per cent., and the nickel content not less than 
3.25 per cent. 



326 



STEEL AND ITS HEAT TREATMENT 



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NICKEL STEELS 



327 



The relation which the maximum size of section bears to the 
physical properties of steel is well illustrated by the following speci- 
fication for Railway Motor Shafts — the steel to contain 3 to 4 per 
cent, nickel: 



Max. Dia., 


Tensile Strength, 


Elastic Limit, 


Elongation, Per 


Reduction of 


Inches. 


Lbs. per Sq. In. 


Lbs. per Sq. In. 


Cent, in 2 Ins. 


Area, PerCent. 


3 


95,000 


65,000 


21 


50 


6 


90,000 


60,000 


22 


50 


10 


85,000 


55,000 


24 


45 


20 


80,000 


45,000 


25 


45 


over 


80,000 


45,000 


24 


40 



The following equations connecting maximum strength, Brin- 
ell hardness number and scleroscope hardness number have been 
computed x from several hundred tests made with nickel steels 
of different carbon content and heat treated to bring out all pos- 
sible physical properties: 

(1) Jlf = 0.71 5-32. 

(2) M = 3.5 S-Q. 

(3) £ = 5.0 £+48. 

M = maximum strength in units of 1000 lbs. per sq. in. 
B = ihe Brinell hardness number. 
$ = the scleroscope hardness number. 

The maximum strength corresponding to different Brinell val- 
ues as determined by equation (1) for these steels is as follows: 



Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


100 
150 
200 
250 
300 


39,000 

74,000 

110,000 

145,000 

181,000 


350 
400 
450 
500 
550 


216,000 
252,000 
287,000 
323,000 
358,000 



The maximum strength corresponding to different scleroscope 
values as determined by equation (2), and the corresponding Brin- 

X R. R. Abbott, A. S. T. M., Vol. XV, Part II, 1915, p. 43 et seq. 



328 



STEEL AND ITS HEAT TREATMENT 



ell numbers as determined by equation (3), for these steels, are as 
follows : 



Scleroscope. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


20 
30 
40 
50 
60 
70 
80 
90 
100 


64,000 
99,000 
134,000 
169,000 
204,000 
239,000 
274,000 
309,000 
344,000 


148 
198 
248 
298 
348 
398 
448 
498 
548 



5 PER CENT. NICKEL STEEL 

The use of nickel steel with the higher nickel content is now 
largely limited to case-hardening purposes, which we have previously 
described. 

The physical results obtained from the treatment of 1-in. 
sections, containing 5 per cent, nickel and 0.33 and 0.43 per cent. 
carbon, are shown in the charts in Figs. 220 and 221 respectively. 



HIGH-NICKEL STEELS 

The high-nickel steels of 25 to 35 per cent, nickel are used prin- 
cipally for gas-engine valves and spindles, ignition and boiler tubes, 
and for other similar purposes. These nickel steels are extremely 
tough, dense, have a high resistance to shock, a low coefficient of 
expansion, and — in particular — are little subject to corrosion. 

Their physical properties in the natural condition may be given 
as follows: 

25 to 28% Nickel 30 to 35% Nickel 
(0.3 to 0.5% Carbon) (average) 

Tensile strength, lbs. per sq. in. . . 85,000 to 92,000 95,000 

Elastic limit, lbs. per sq. in 35,000 to 50,000 50,000 

Elongation, per cent, in 2 ins. . . 30 to 35 40 

Reduction of area, per cent .... 50 to 60 58 

These steels do not respond to heat treatment, but may be an- 
nealed at about 1450° F. to facilitate machining, after which the 



NICKEL STEELS 



329 





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Chemical Analysis: Critical Ranges: Size of Seotion: Treatment: 

C. 0.33 Acl 1276° 1 inch round Previously double 
Mn. 0.40 Ac2.3 1330° quenched in Oil 
P. low Ac3.2 1160° and annealed. 
S. low Arl 1085° Quenched in Oil 
Si. 0.15 from 1375f and 
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330 



STEEL AND ITS HEAT TREATMENT 





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NICKEL STEELS 331 

physical properties of the 30 to 35 per cent, nickel steels given above 
will average: 

Tensile strength, lbs. per sq. in 85,000 

Elongation, per cent, in 2 ins 30 

Reduction of area, per cent 40 

Nickel steels with 35 to 38 per cent, nickel and 0.3 to 0.5 per cent, 
carbon have a coefficient of expansion which is less than any metal 
known and amounts practically to zero. These alloys are used for 
various clock, geodetic and similar instruments for precise measure- 
ments. The physical properties of these steels are approximately: 

Tensile strength, lbs. per sq. in 100,000 to 115,000 

Elastic limit, lbs. per sq. in 64,000 to 78,000 

Elongation, per cent, in 2 ins 35 to 25 

Reduction of area, per cent 50 



CHAPTER XIV 

CHROMIUM STEELS 

Chromium in steel has the characteristic function of opposing 
both the disintegration and reconstitution of cementite. 1 This is 
made noticeable by the changes in the critical ranges of the steel, as it 
makes them take place more slowly, that is, it has the tendency to 
raise the Ac range and to lower the Ar range. The action of chro- 
mium in raising the critical range on heating is illustrated clearly by 



15C0 



1450 



feUOO 



1350 



1300 




1.5" 2.0 2.5 

Per Cent Chromium. 



Fig. 222. — Acl Critical Range of 0.5 Per cent. Carbon Steel with Increasing 

Chromium Content. 



Fig. 222, which gives the position of the Acl critical range of steel 
containing 0.50 per cent, carbon, 0.50 per cent, manganese and with a 
chromium content increasing from to 4 per cent. 

Each 0.1 per cent, of chromium, up to at least 4 per cent., raises 
the Ac temperature by about 3° F. 

1 Savoia, " Metallography " (trans.), p. 315. 
332 



CHROMIUM STEELS 



333 



Chromium steels are, therefore, capable of greater hardness 
because rapid cooling is able more completely to prevent the decom- 
composition of the austenite. 1 The greater hardness of chromium 
steels is also due to the presence of double carbides of chromium and 
iron in the steel in the hardened or slightly tempered condition. 
This additional mineral hardness is obtained without raising the 
brittleness to such a degree as does carbon. The degree of hardness 
is within certain limits dependent upon the carbon content, as 
chromium alone will not harden iron. Harbord 2 states that car- 
bonless, or nearly carbonless, chromium steel does not harden when 
water quenched. Toughness is also conferred by the degree of fine- 
ness of the structure, which is a characteristic of chromium steels 
(similar to that of nickel), thus increasing the tensile strength and 
elastic limit without a noticeable loss of ductility. Thus we have 
that condition of " tough-hardness " which makes chromium steel 
so valuable in parts requiring great resistance to wear (abrasive 
action 3 ). In regard to corrosion, Chappell 4 states that "in neu- 
tral corroding media the resistance offered to corrosion apparently 
rises with the percentage of chromium. This is particularly the case 
for salt water, and the employment of chromium steels in the con- 
struction of ships would appear to be fully justified on this ground 
alone. — The corrosion factor does not appear to be a purely addi- 
tive quantity." 

The influence of chromium towards increasing the brittleness of 
steel, especially upon prolonged heating at high temperatures in the 
case-hardening process, is shown in the following results of tests by 
Guillet: 5 



Treatment. 


Resistance to Shock. 


Chromium 0.70% 
Carbon 0.05% 


Chromium 1.20% 
Carbon 0.05% 


Annealed 

Quenched 

Heated for 4 hours at 1830° F. . 
After double quenching 


32 kgms. 

22 

5 

26 


25 kgms. 
25 kgms. 
5 
20 



1 Stoughton, " Metallurgy of Steel," p. 407. 

2 Harbord, " Metallurgy of Steel," Vol. I, p. 390. 

3 See F. Robin, Journal Iron and Steel Inst., Vol. II, 1910. 

4 Chappell, Journal Iron and Steel Inst., 1912. 

5 L. Guillet, " Trempe, recuit, revenu — " 



334 STEEL AND ITS HEAT TREATMENT 

0.5 PER CENT. CHROMIUM STEELS 

Low-chromium steels find many valuable uses and at only a 
slightly increased cost, since the usual charge by open-hearth steel 
manufacturers for 0.5 per cent, chromium steel is only about one or 
two dollars a ton over the " base price." One-half of one per cent, 
chromium raises the critical range on heating by about 15° F. over 
that of the corresponding straight carbon steel. As a leeway of at 
least 50° F. over the critical range is usually allowed for in hardening, 
these chromium steels may in general be hardened at the same tem- 
perature as for straight carbon steels of the same carbon content. 

0.5 CHROMIUM, LOW CARBON 

For carbons up to about 0.35 per cent, the addition of this small 
amount of chromium confers practically no additional physical prop- 
erties other than those which might be obtained by the use of a 
slightly higher carbon content in a straight carbon steel. On the 
other hand, for case-hardening purposes this small amount of chro- 
mium confers homogeneity, greater strength and wearing qualities, 
due to the much finer grain throughout after the double quenching, 
and to the presence of double carbides in the case. It should be 
remembered that chromium strengthens the cementitic element of 
the structure of steel, which in turn must depend upon the amount 
of pearlite; nickel, on the other hand, influences the ferrite con- 
stituent. Although both elements will tend to make the structure 
much finer, it is evident that while nickel will have its greatest effect 
upon the lower carbon steels (those containing large amounts of 
ferrite) , chromium will be of the most importance in the high carbons 
in which there is considerable cementite. Thus it is that although 
chromium in small amounts will be of little direct importance in 
ordinary heat-treated low-carbon steels, it will be of tremendous 
importance in any case-hardening operation which will produce a 
high-carbon case. 

The addition of chromium probably increases the velocity of pene- 
tration of the carburization, under identical conditions, over that 
of the corresponding straight carbon steel. With greater amounts 
of chromium there is also the tendency toward a higher maximum 
carbon concentration than that obtained with carbon steels similarly 
carburized. The tendency to surface oxidation during carburiza- 
tion as a characteristic of chromium steels has been noted by sev- 



CHROMIUM STEELS 335 

eral investigators; methods involving the use of mixed cements 
may be used, however, which will modify or even eliminate this 
action. 

As chromium emphasizes the harmful effects of prolonged heating 
(opposite to the action of nickel), it is always necessary to double 
quench the steel; that is, a regenerative quenching as well as the 
usual hardening quenching. The greater surface hardness obtained 
by the use of chromium steels permits the use of oil for both quench- 
ings, if desired, and thus tends to avoid deformation of the steel. | 

0.50 chromium — 0.35 to 0.50 carbon 

With a carbon content up to say 0.50 per cent, carbon, the addi- 
tion of a half per cent, chromium will give, after heat treatment, 
about 15 per cent, increase in tensile strength, 10 per cent, increase 
in elastic limit, with practically no loss in ductility, over straight 
carbon steels of the same carbon content. In the hardened condition 
it gives excellent service for wearing surfaces, such as the jaws of 
wrenches, small gears, etc. The tables on page 336 give test results 
obtained from open-hearth steels. 

0.50 CHROMIUM, OVER 0.50 CARBON 

With the increase in carbon the influence of chromium becomes 
even more marked, due to the increasing amounts of double carbide. 
The hardness increase is greater proportionally than the carbon 
increase. For well-bits and jars in the hardened condition this steel 
has no equal among the low-priced alloys or straight carbon steels. 
For die blocks used in drop-forging work it does not seem quite to 
" hit the mark," apparently not having the requisite toughness 
to offset the brittleness, especially in the larger sections. With 
0.70 to 0.80 per cent, carbon it makes an excellent chisel, while with 
0.90 to 1.00 per cent, carbon and about 0.60 per cent manganese it 
gives even better results for pneumatic chipping chisels than do 
many varieties of high-speed steel. The Germans in particular 
have made great use of 0.5 per cent, chromium steels for tools, such 
as drills, saw-blades, knives, razors, files, and similar tools requir- 
ing a keen cutting edge. Further increase in hardness may also 
be obtained by the addition of silicon and manganese. The mini- 
mum hardening temperatures for the higher carbons should always 
be used to obtain the maximum effect of the chromium. 



336 



STEEL AND ITS HEAT TREATMENT 



Treatment. 


C. 


Mn. 


P. 


S. 


Si. 


Cr. 


1500° F. oil/1200° F.* 

1500° F. oil/1300° F.* 

1500° F. oil/1300° F.* 

As rolled, 4"X4" billet.. . 


0.36 

0.40 
0.47 


0.44 

0.50 
0.60 


0.008 

0.015 
0.006 


0.021 

0.025 
0.025 


0.05 


0.57 
55 


1460° F. water/1000* . . 
1460° F. water/1100*. . 
1460° F. water/1200*. . 
1460° F. water/1300*. . 


0.108 


0.51 




0.47 


0.55 


0.015 


0.029 




0.57 








Treatment. 


Tensile 
Strength, 
Lbs. per 

Sq. in. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation, 
Per cent, 
in 2 Ins. 


Reduc- 
tion of 
Area. 
Per cent. 


Brinell 
Hard- 
ness. 


Sclero- 
scope 
Hard- 
ness. 


1500° F. oil/1200° F.* 
1500° F. oil/1300 F.* 
1500° F. oil/1300 F.* 
As rolled, 4"x4" billet. . 
1460° F. water/1000* . . 
1460° F. water/1100*. . 
1460° F. water/1200* . . 
1460° F. water/1300*. . 


103,200 

99,450 

94,100 

93,000 

168,420 

143,060 

112,930 

120,550 


76,000 

71,240 

67,460 

72,000 

153,720 

128,860 

120,750 

109,540 


27.0 
22.0 
28.0 
26.0 
19.0 
19.0 
22.0 
25.0 


62.0 
51.4 
69.0 
50.0 
52.6 
60.2 
60.5 
67.6 


311 

277 
262 
212 


44 
36 
35 
30 


8" round as hammered . . 


95,000 


50,000 


22^0 


50.0 







* Tests from 1-in. rounds. 

Critical range diagrams are shown in Figs. 223 and 224. 



Results obtained upon oil quenching from 1400° F. and subse- 
quent toughening of 1-in. rounds of the approximate composition 
of 0.70 per cent, carbon, 0.60 manganese and 0.50 chromium are 
given in the following table; note especially the high proportion 
of the elastic limit to the tensile strength, combined with good duc- 
tility : 



Toughening 


Tensile Strength, 


Elastic Limit, 


Elongation, Per 


Reduction of 


Temperature, °F. 


Lbs. per Sq. In. 


Lbs. per Sq. In. 


Cent, in 2 Ins. 


Area, Per Cent. 


900 


199,500 


179,050 


6.0 


25.6 


1000 


168,900 


143,500 


12.5 


33.8 


1100 


145,700 


119,400 


15.0 


42.2 


1200 


120,000 


105,000 


17.0 ' 


47.5 


1300 


107,100 


91,400 


22.0 


58.1 



The critical range diagram is shown in Fig. 225. 
1.5 PER CENT. CHROMIUM STEELS 

Chromium steels with about 1.00 to 1.75 per cent, chromium, with 
high carbon, find their greatest use in balls, ball-races, cones, roller 



CHROME STEELS 



337 




Fig. 223. — Critical Range Diagram of Heat 2101. Carbon, 0.47 per cent.; 
Manganese, 0.60 per cent.; Phosphorus, 0.006 per cent.; Sulphur, 
0.025 per cent.; Silicon, 0.108 per cent.; Chromium, 0.51 per cent. 




Fig. 224. — Critical Ranges of Basic Open-hearth Steel, Heat 8148. Carbon, 
0.50 per cent.; Manganese, 0.49 per cent.; Phosphorus, 0.010 per 
cent.; Sulphur, 0.026 per cent.; Silicon, 0.05 per cent.; Chromium, 
0.57 per cent. 



338 



STEEL AND ITS HEAT TREATMENT 




Fig. 225. — Critical Range Diagram of Chromium Carbon Steel. Carbon, Approx. 
0.70 per cent.; Manganese, Approx. 0.60 per cent.; Chromium 
Approx. 0.50 per cent. 



bearings, crushing machinery, safe steel, tools, and other parts 
requiring a very hard surface. The use of about 1 per cent, of carbon 
combined with the above chromium content appears to give the 
highest combination of " tough-hardness " and plasticity. 

The original ingot structure of a steel containing approximately 
1.1 per cent, carbon and 1.75 per cent, chromium is shown in Fig. 226 
and at the left in Fig. 227. The beautiful, frost-like, curved patterns 
of the photomicrograph represent the columnar crystals of the outer 
part of the ingot, and formed during the solidification of the steel. 
It will be noticed also that the carbide is almost entirely in solution. 
The Brinell hardness of this steel ingot was 275. 

Such steel requires care in forging, which must be done at 
good red heat and with powerful blows. The effect of " welding " 
the above 8-in. square ingot into a 6-in. square billet is illustrated 
by Fig. 228. The high heating and the working of the metal have 
broken up the dendritic ingot structure to some extent, but more 
particularly have brought about a smaller grain size recognizable 
by the vein-like outlines of carbide. Additional working further 
refines the steel, entirely breaking up the coarse ingot structure, 
but, nevertheless, leaving the steel in a condition too hard for efficient 
machine work. Annealing, therefore, must be used to soften the 
steel. 

Annealing. — The degree of hardness and machineability depend 



CHROMIUM STEELS 



339 



largely upon the distribution of the chromium-iron carbide; this, in 
turn, is controlled by the temperature-time elements in the heating 
and cooling phases. Thus it has been shown that in the ingot the 




Fig. 226.- 



-1.1 Per cent. Carbon, 1.75 Per cent. Chromium, 8 In. Sq. Ingot. 
X100. (Bullens.) 




Fig. 227. — 1.1 Per cent. Carbon, 1.75 Per cent. Chromium Steel, 
and Annealed Structure (right). 



Ingot (left) 



carbide is held in solution and has been precipitated only slightly 
by the high heating and comparatively rapid cooling incident to the 
hammering operation. 

The effect of high and low heating, followed by slow cooling, is, 
aside from the increase in grain size, to precipitate further the excess 



340 



STEEL AND ITS HEAT TREATMENT 




Fig. 228.— Steel of Fig. 226 Welded to 5-in. Square Bar. X100. (Bullens.) 




Fig. 229. High Temperature Anneal. X100. (Bullens.) 



CHROMIUM STEELS 



341 



carbide to the grain boundaries as is shown in Figs. 229, 230 and 231. 
These are photomicrographs of the same steel but at different mag- 




Fig. 230.— Same as Fig. 229, but X300. (Bullens.) 




Fig. 231.— Same as Fig. 229, but X600. (Bullens.) 



342 



STEEL AND ITS HEAT TREATMENT 



nifications, and illustrate the steps in the spheroidizing of the carbide. 
The central portions of the grains are composed of sorbitic or lamellar 
pearlite, as is shown by the high magnifications. Some of the excess 



1400 



1200 



loeo 



800 



/// 


V/////////////////y 


'////////////>. 




• 












f 










J_ 











25 50 75 100 Hours 

Fig. 232. — Indicated-temperature Annealing Chart, 1.1 Per cent. Carbon, 
1.75 Per cent. Chromium Steel. 

carbide appears as veins at the grain boundaries, but the greater 
portion has been dissociated more completely into nodules or spheroids. 
In other words, the slow cooling from a high temperature through 




Fig. 233.— Chromium Steel Annealed. X100. (Bullens.) 

the critical range, or under-cooling, has been sufficient to speroid- 
ize largely the excess carbide, but has not been sufficient to break up 
the pearlite. This steel, having a Brinell hardness number of 230 
has been annealed improperly, has too coarse grains, and is too hard 
for efficient machining. 



CHROMIUM STEELS 



343 



In order that this high chromium steel may be readily machine- 
able, it has been found that the eutectoid carbide as well as the 
excess carbide must be spheroidized. This may be accomplished by 
a very long heating at a temperature at or slightly above the critical 
range, followed by a very gradual and extremely slow cooling through 
the critical range. The elements of time, mass and surface enter 
very strongly into this phase of annealing work, especially in cases 
where the annealing of large tonnages is required. The tempera- 
tures to be used for steel for ball- and roller-bearings having a chem- 




Fig. 234.— Same as Fig. 233, but X300. (Bullens.) 



ical composition of 0.9 to 1.1 per cent, carbon and 1.25 to 1.75 per 
cent, chromium will vary between 1400° and 1475° F.; the time oi 
saturation will depend upon the mass and exposed surface of the 
charge and will usually take several days. Such steel in the soft 
annealed condition should have a Brinell hardness of not over 170. 
The heat log of this steel satisfactorily annealed to give a Brinell 
hardness of 149 is shown in Fig. 232. A comparison of the fractures 
of the ingot and this annealed steel is illustrated by Fig. 227, while 
the microstructure of the annealed steel is shown in Figs. 233 and 
234. The photomicrographs show that at least the major part of 
the carbide has been spheroidized. 



344 



STEEL AND ITS HEAT TREATMENT 



An expeditious method for annealing this steel — especially for small 
lots — is to normalize and then anneal, as follows : first . thoroughly 
heat the steel at a temperature above the Acm point for the " solu- 
tion " of the cementite, air cool to a temperature beneath that of the 
Ar point to prevent disintegration, reheat to a temperature slightly 
over the Ac 1.2.3 point to refine the grain, and slowly cool in the 
furnace or in lime to obtain the maximum degree of ductility (soft- 
ness). For a steel containing 0.9 to 1.1 per cent, carbon, under 0.50 
per cent, manganese, and about 1.00 per cent, chromium, the fol- 
lowing temperatures may be used to advantage: 

1. Heat to 1700° to 1750° F. 

2. Air cool to about 800° F. 

3. Heat to 1425° to 1450° F. 

4. Slow cool in furnace or in lime. 

Note. — Add 25° to the temperature (1) and (3) if the chromium 
is up to 1.75 per cent. 

Hardening. — These steels take on great hardness, both on the 
surface and at depth, when hardened, for which either water or 
oil may be used. In the hardened condition the Shore scleroscope 
gives a hardness figure of about 100. The critical ranges for these 
steels with over 0.90 per cent, carbon will vary from 1360° to 1375° 
F. for 0.5 per cent, chromium, to 1400° to 1425° for 1.5 per cent, 
chromium. The results obtained from the heat treatment of 1-in. 
rounds of a 0.64 per cent, carbon chromium steel are as follows: 

0.64 Carbon; 0.2S Manganese; 0.17 Silicon; 1.04 Chromium 



Quenched in Oil 

from 1600° F. 

and Toughened 

at - Deg. F. 


Tensile 

Strength, 

Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elongation, 
Per Cent, 
in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


Brinell 
Hardness. 


750 

930 

1110 


227,500 
212,000 
186,000 


170,000 
155,000 
127,500 


5.0 

8.0 

10.0 


13.5 
19.5 

22.5 


477 
444 
387 



2.00 PER CENT CHROMIUM STEELS 

The tables on page 345, taken from the work of McWilliams and 
Barnes, 1 and rearranged, show the physical properties of 2 per 
cent, chromium steels of ascending carbons as rolled, heat treated and 
annealed. Chromium steels with about 2 per cent, of chromium are 

1 " Iron and Steel Inst. Jour." 



CHROMIUM STEELS 



345 



2.00 Chromium — 0.20 Carbon. 


Critical Range Ac 3 = 


1512° F. 


Treatment. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation, 

Per Cent. 

in 2 Ins. 


Reduc- 
tion of 
Area, 
Per Cent. 


Alterna- 
tions 
(Ar- 
nold's). 


1. As rolled 


70,400 

137,200 

116,000 

82,400 

66,000 


45,600 

134,000 

110,000 

64,000 

32,000 


30.5 
12.5 
16.0 
28.0 
40.5 


71.2 
40.6 
50.7 

70.2 
77.9 


331 


2. 1475° F. in water/ 750° F. 

3. 1475° F. in water/1025 F. 

4. 1475° F. in water/1300 F. 

5. Annealed 


96 
144 
234 
410 







2.00 Chromium — 0.25 Carbon. 



1. As rolled 

2. 1475° F. in water/ 750° F. 

3. 1475° F. in water/1025 F. 

4. 1475° F. in water/1300 F. 

5. Annealed 



Critical Range Ac 3 = 1490° F. 

312 
103 
99 
204 
437 



67,200 


48,800 


30.0 


68.4 


176,400 


156,600 


12.0 


42.5 


144,000 


136,000 


14.5 


51.5 


96,000 


82,000 


25.0 


68.6 


70,000 


32,000 


39.5 


73.8 



2.00 Chromuim — 0.32 Carbon. 

1. As rolled 

2. 1475° F. in water/ 750° F. 

3. 1475° F. in water/1025 F. 

4. 1475° F. in water/1300 F. 

5. Annealed 



Critical Range Ac 3 = 1445° F. 

355 

94 
141 
197 

482 



92,600 


60,000 


26.0 


65.4 


200,000 


184,000 


9.5 


37.0 


159,200 


151,800 


15.0 


52.2 


109,600 


94,000 


22.5 


67.2 


60,800 


28,800 


37.0 


70.7 



2.00 Chromium — 0.50 Carbon. 

1. As rolled 

2. 1475° F. in water/ 750° F. 

3. 1475° F. in water/1025 F. 

4. 1475° F. in water/1300 F. 

5. Annealed 



Critical Range Ac 3 = 1432° F. 

378 
88 
111 
169 
440 



107,600 


64,000 


20.5 


65.8 


228,200 


224,000 


9.0 : 


30.3 


179,200 


170,200 


13.0 : 


42.5 


124,800 


114,000 


21.0 ! 


61.5 


75,200 


25,400 


28.0 ! 


55.4 



2.00 Chromium — 0.65 Carbon. 



Critical Range Ac 3 = 1440° F. 



1. As rolled 

2. 1475° F. in water/ 750° F. 

3. 1475° F. in water/1025 F. 

4. 1475° F. in water/1300 F. 

5. Annealed 



142,600 

193,000 

125,200 

97,800 



116,000 

188,200 

113,600 

64,000 



14.5 

10.0 
21.0 
21.5 



41.1 

32.4 
55.6 

62.2 



292 

74 
133 
214 



2.00 Chromium — 0.85 Carbon. 



As rolled 

1475° F. in water/ 750° F. 



3. 1475° F. in water/1025 F. 



1475° F. in water/1300 F. 
Annealed . 



151,800 

191,400 

126,000 

80,200 



Critical Range Ac 3 = 1430° F. 



104,000 

185,000 

115,000 

37,600 



10.0 

8.5 
20.0 
32.0 



18.3 

28.2 
51.7 
63.5 



178 

65 
155 
316 



346 



STEEL AND ITS HEAT TREATMENT 



largely used in the manufacture of armor-piercing projectiles, besides 
in such objects which require an extremely hard-wearing surface 
such as in crushers, cold rolls, drawing dies, special files, etc. 



HIGH-CHROMIUM CARBON STEELS 



For general practical purposes and heat-treatment work the 
chromium content is limited to that percentage below which the steel 




Double Carbide 



Pearlite 



5&Cr. 



%G. 1.6L' 

Fig. 235. — Microscopic Constituents of Chromium Carbon Steels. 



2.30 



as cast will be pearlitic — that is, the critical temperatures on cool- 
ing are all above atmospheric temperatures and the steel structure 
is composed of pearlite plus either ferrite or cementite. The hard- 
ness increases with the chromium content, and, according to Arnold 
and Read, appears to be independent of the carbon content, whilst 
the brittleness is far less than in carbon steels of the same carbon 
content. 

Martensitic Steels. — When the chromium content reaches from 
5 to 7 per cent., dependent upon the carbon, the change point, Ar, 
will fall below normal "temperatures and the structure will become 



CHROMIUM STEELS 



347 



troostitic or martensite. That is, the structure will be comparable 
with that of a straight carbon steel in the hardened condition. These 
steels have high tensile strength and elastic limit, low ductility, 
great hardness and medium brittleness. Heat treatment has little 
or no influence, except, perhaps, to refine the grain. On account of 
their physical characteristics these steels are but little used, except 
as applied to special tool steel. As the chromium content is again 
increased to about 12 to 15 per cent., intensely white grains of the 
double carbide of chromium and iron form within the martensite, and 
gradually occupy the whole field with further increase of chromium. 
These structural changes for varying percentages of chromium, and 
with 0.2 and 0.8 per cent, carbon respectively, are given by Guillet as 
follows : 



Structure With 0.2 Carbon 

Pearlitic . to 7 per cent. Cr. 

Troostitic 7 to 8 per cent. Cr. 

Martensitic 8 to 13 per cent. Cr. 

Martensite plus double carbide 13 to 20 per cent. Cr. \ 
Double carbide over 20 per cent. Cr. J 



With;0.8 Carbon 
to 5 per cent. Cr. 
5 to 18 per cent. Cr. 

18 and over per cent. Cr. 



These changes are shown graphically in Fig. 235. 

The effect of annealing and heat treatment upon high-chromium 
carbon steels, with approximately 0.4 per cent, carbon is given in 
the following table: 1 

Heat Treatment of High-chromium Steels, 0.4 Carbon 



Chrom- 
ium, 
Per cent. 


Treatment. 


Tensile 

Strength, 
Lbs. per 
Sq. In. 


Elastic 

Limit 

Lbs. per 

Sq. In. 


Elon- 
gation 

Per cent. 

in 2 Ins. 


Reduction 
of Area, 
Per cent. 


5 
10 
15 
20 


Annealed 

Hardened & tempered 

Annealed 

Hardened & tempered 

Annealed 

Hardened & tempered 

Annealed 

Hardened & tempered 
Annealed 


53,760 

123,648 

94,080 

121,632 

101,472 

130,144 

80,864 

90,272 

94,526 

90,496 

93,184 

87,360 


39,872 
109,312 
51,296 
94,976 
56,896 
109,312 
47,488 
61,824 
66,752 
61,824 
71,232 
64,518 


24 

12 

21.5 

12 

18.5 

11.5 

21.5 

19.5 

18 

20 

19 

19 


24.0 
37.0 
44.0 
53.6 
50.0 
54.6 
46.5 
51.5 
62.1 


25 


Hardened 


50.0 




Annealed 


62.0 


30 


Hardened. . . .' 


65.0 









1 J. Holtzer & Cie., Loire, France, from Harbord's " Metallurgy," I, 391. 



348 STEEL AND ITS HEAT TREATMENT 

Further data on high-chromium steels may be obtained from the 
researches of Guillet, 1 Portevin, 2 Arnold and Read/ 3 Becker, 4 Mars, 5 
and others. 

The influence of chromium as used in high-speed steels is described 
in the chapter on High-speed Steels. 

1 Guillet, " Les Aciers Speciaux." 

2 A. Portevin, " Revue de Metallurgie," 1909, No. 12, p. 1264, " Metal- 
lurgies 1910, Heft 6, S. 177. 

3 Arnold and Read, " Iron and Steel Inst. Journ." 

4 O. M. Becker, " High-speed Steel." 
6 Mars, " Spezialstahle." 1912. 



CHAPTER XV 

CHROMIUM NICKEL STEELS 

Chromium Nickel vs. Chromium Vanadium Steels. — Chromium 

nickel steels, as a type composition, probably represent the best all- 
around alloy steels in commercial use for general purposes. By this 
it is not to be inferred that chromium nickel should always be used in 
preference to other alloys; as a matter of fact, each type is more or 
less peculiarly adapted to work of a distinctive nature. On the other 
hand, chromium nickel steel of suitable composition will satisfy nearly 
every condition for structural and similar purposes. Much has been 
said and done with chromium vanadium steels, and while the latter 
undoubtedly do fill a long-felt want along certain lines, it should not 
be said that chromium vanadium steels are superior to chromium 
nickel steels. In fact, with a few exceptions, chromium nickel steels 
of suitable composition will generally measure up to any standards 
set by the ordinary vanadium alloys and at equal or at even less cost. 
Neither chromium vanadium, nor chromium nickel, nor any one 
type of steel is a general prescription for the every ill of the steel user : 
each steel has its distinctive characteristics and applications. And 
notwithstanding the mass of advertising " literature " to the con- 
trary, it would also be decidedly improper to state, as a general rule, 
that either is superior to the other. 

Influence of Chromium and Nickel. — Chromium nickel steels of 
suitable composition appear to have the beneficial effects of both the 
chromium and nickel, but without the disadvantages which are 
inherent in the use of either one separately. Moreover, the presence 
of both chromium and nickel seems to intensify certain physical 
characteristics. To the increased ductility and toughness conferred 
by nickel on the ferrite there is added the mineral hardness given to 
the cementite and pearlite by the chromium, but with a greater 
resultant effect. Again, while the addition of nickel alone serves to 
diminish the susceptibility to brittleness in the steel upon pro- 
longed heating or sudden cooling — in comparison with the corre- 
sponding straight carbon steels — and, on the other hand, the presence 

349 



350 STEEL AND ITS HEAT TREATMENT 

of chromium alone tends to the opposite effect, a suitable combina- 
tion of the two alloying elements tends to neutralize the harmful 
effects and also to magnify the good points. This is not only brought 
out in the static strength and ductility, but also in the dynamic 
strength or fatigue resistance. 

Statements have frequently appeared in print to the effect that 
nickel " poisons " the steel dynamically; that chromium has little 
influence one way or the other upon the fatigue resistance; and that 
chromium nickel steels are inferior along these lines to certain other 
specific alloy steels. In considering these broad statements there 
are three things in particular which should be noted. Firstly, that 
in the present state of the art of dynamic strength testing, the 
results so obtained are often widely divergent for the same steel, not 
to mention any comparison of results upon different steels of dis- 
similar type. Secondly, even assuming that concordant and strictly 
comparative results could be thus obtained by means of the testing- 
machines now in use,. the majority of the experimental results put 
forth to prove the general inferiority (dynamically) of chromium 
nickel steels in relation to certain other types (e.g., chromium vana- 
dium) are oftentimes not really comparative at all, since the two 
distinctive types of steel have been heat treated alike. That is, 
while it may be perfectly good practice to quench a chromium vana- 
dium steel from say 1650° F., it might be distinctively poor practice 
to quench a chromium nickel steel from the same temperature. And 
yet many " comparative " results have been obtained in just such a 
manner, to the detriment of either one steel or the other. Rather, 
then, should each steel be treated in that impersonal and strictly 
scientific manner which will tend to bring out the maximum qualities 
of each; and then should the tests be made upon the same machine 
under like conditions. Thirdly, whatever may be the influence of 
chromium or nickel alone upon the dynamic strength of steel, it has 
been repeatedly demonstrated that the proper combination of the two 
alloys undoubtedly produces a type of metal with vastly improved 
capacity for resistance to fatigue. 

Commercial Ratio of Chromium and Nickel Content. — From 
experience in both the manufacture and use of chromium nickel 
steels it would appear that there is some ratio existing between 
the proportion of the chromium and nickel which will give the most 
efficient combination of physical characteristics. In other words, 
by combining the chromium and nickel in some such ratio, the less 
susceptibility to brittleness upon prolonged heating which is char= 



CHROMIUM NICKEL STEELS 



351 



acteristic of nickel additions will modify the greater susceptibility 
to brittleness which is given by chromium alone, giving a stronger 
and better steel than may be obtained when this ratio is not ob- 
served. Again, it will be observed that if the chromium content 
greatly exceeds a certain proportion in respect to the nickel, the steel 
will be more difficult to heat treat successfully, the temperature limits 
are more narrow, and the possibility of poor results is greatly in- 
creased. This best ratio is probably about 2\ parts of nickel to 
about 1 part of chromium. Thus we have the principal standard 




Fig. 236.— Protective Deck Steel. (Bullens.) 



types of 3.5 nickel and 1.5 chromium, 1.5 nickel and 0.6 chromium, 
and various intermediate types. 

Carburization. — The carburization of chromium nickel steels does 
not differ in principle from that previously described. These steels 
generally carburize more rapidly and better than straight carbon 
steels, and, in particular, give the characteristic gradual cemented 
zone which should always be aimed for. The presence of suitable 
proportions of chromium and nickel, as previously mentioned, also 
gives that low brittleness of core which is so desirable; this fact 
even permits the use of steels up to some 0.3 per cent, carbon without 



352 STEEL AND ITS HEAT TREATMENT 

great danger. The use of chromium nickel steel in case-hardening 
work covers a wide range — from small gears subject to great shock 
and wear to the heaviest grades of armor plate. 

Heat Treatment. — The heat treatment of these steels does not 
present any new problems. In the main the discussion under the 
chapters on Carbon Steels and Nickel Steels will apply equally well 
to chromium nickel steels. Similarly to nickel steels, these steels are 
less susceptible to the deleterious influence of high temperatures, 
and which will be subsequently mentioned. Suitable heat treatment 
will develop a very fine micro-structure, as is shown in Fig. 236, 
representing the structure of specially treated chromium nickel steel 
used for protective deck plate on battleships; the physical proper- 
ties on this particular steel were: 

Tensile strength, lbs. per sq. in 132,000 

Elastic limit, lbs. per sq. in 116,700 

Elongation, per cent, in 2 ins , 23 

Reduction of area, per cent 64 

Proper annealing will likewise develop a good micro-structure 
in the steel, as is shown in Fig. 114. The critical ranges of chromium 
nickel steels are somewhat lower than those of the corresponding 
straight carbon steels, so that lower temperatures may be used for 
quenching. 

In general, the best treatments which can be given to these alloy 
steels after forging are as follows: 



a. Quench in oil from about 175° to 200° F. over the critical 

range. 
6. Quench in oil from about 50° over the critical range. 

c. Anneal at about 75° under the critical range (also see II). 

Machine. 

d. Quench in the proper medium from about 50° over the 

range. 

e. Draw the temper to suit the work in hand. 

II 

For shafts and other structural parts in which the desired physical 
properties may be obtained by a drawing temperature of about 900° 



CHROMIUM NICKEL STEELS 353 

F. or over, and which will leave the steel in a machinable condition, 
Treatment I may be modified at (c) as thus noted, and no further 
treatment will be required. But if the drawing temperature must 
be much lower, as for gears, the full treatment as in (I) is advisable. 

a. Quench in oil from about 175° to 200° F. over the critical 

range. 

b. Quench in oil from about 50° over the critical range. 

c. Draw at 900° or more, as the work may require. Machine. 



Ill 

The full treatment as given under (I) may be modified, if 
desired, to the following, for parts to be drawn below 900° or 
1000° F.: 

a. Quench in oil from about 175° to 200° over the critical 

range. 

b. Reheat to about 25° to 50° F. over the critical range and 

cool slowly. Machine. 

c. Quench in oil from about 50° over the critical range. 
cL Draw to the temperature required by the work. 



LOW CHROMIUM NICKEL STEELS 

The low chromium nickel steels, containing approximately 0.5 
per cent, chromium and 1.5 per cent, nickel, are the most used of all 
the chromium nickel alloys. After forging or rolling, this grade of 
synthetic steel may be heat treated to develop physical character- 
istics nearly equivalent to a 3.5 per cent, nickel steel of similar car- 
bon content. It does not have the objectionable tendency to lam- 
inate which may characterize the latter steel, and on account of the 
less cost of alloys, this chromium nickel steel is sold at a price con- 
siderably lower than that of 3.5 per cent, nickel steel. This grade 
of chromium nickel steel forges well and machines easily, does not 
require the more narrow temperature limits in heat treatment as 
do some steels containing a larger, although riot as well proportioned, 
amount of chromium and nickel. 

The physical tests obtained from heat-treated steel of 1-in. 
sections, containing approximately 0.5 per cent, chromium and 1.5 
per cent, nickel, are given in the charts in Figs. 237 to 240. 



354 



STEEL AND ITS HEAT TREATMENT 





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CHROMIUM NICKEL STEELS 



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Chemical Analysis: Critical Range.: Size of Section : Treatment: 
C. 0.28 Acl 1320° 1 inch round Quencliedin 
Mn. 0.19 Ac3 1100° water from 
P. 0.015 Arl 1200° 1175° and 
S. 0.021 drawn as 
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^Cccinical Analysis: Critical Rangfp; Size of Section: Treatment: 

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Mn. 0.65 Ac3 1385° quenohed in Oil 
P. 0.02 Ar3 1290° and annealed. 
S . 0.03 Arl 1210° Quenched in Oil 
Si. 0.15 from 1430°and 
Cr. 0.42 drawn aa given. 
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CHROMIUM NICKEL STEELS 



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Chemical Analysis: Critical Ranges: Size of Section: Treatment: 
C. 0.545 Ac '1300°-1320 O 1 incll round Quenched in 
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P. 0.010 1400° and 
S. 0.020 drawn as 
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358 



STEEL AND ITS HEAT TREATMENT 



Small automobile forgings specifying 0.20 to 0.27 per cent, 
carbon (with the above amount of chromium and nickel) may readily 
be heat treated to fulfill the requirements of: 

Tensile strength, lbs. per sq. in 100,000 

Elastic limit, lbs. per sq. in 85,000 

Elongation, per cent, in 2 ins 16 

The critical range of steels of this analysis is about 1425-1450° F.; 
a quenching temperature of 1475° to 1500° will generally give the 
best results in small sections which only require a single quenching. 

An interesting comparison between this grade of chrome nickel 
steel and 3| per cent, nickel steel with the same carbon content is 
shown by the following requirements for automobile axles of a diam- 
eter of 1| ins. as specified by one manufacturer; 



Specification for Heat-treated Automobile Axles, If -in. Diam. 





Chromium Nickel 
Steel. 


Nickel Steel. 


Carbon 


0.30 to 0.40 
0.50 to 0.70 
under . 04 

0.20 

1.50 

0.50 

120,000 

110,000 

16 

45 

180° 


0.30 to 0.40 


Manganese 

Phos. and sul 


0.50 to 0.70 
under . 04 


Silicon 

Nickel 

Chromium 


0.20 
3.50 


Tensile strength, lbs. per sq. in 

Elastic limit, lbs. per sq. in 

Elongation, per cent, in 2 ins 


135,000 

120,000 

16.5 


Reduction of Area, per cent 

Bend test, flat around 


50 
180° 







The following results were obtained from test pieces taken from 
full-size forgings of approximately 8 to 10 ins. in diameter, and 
having an analysis of carbon 0.38 per cent., manganese 0.55 per 
cent., chromium 0.31 per cent., and nickel 1.20 per cent. Many 
interesting points were noticed in the treatment of this grade of 
steel approximating the analysis given, and especially the seeming 
contradiction of the annealing and hardening temperatures. The 
critical range of this steel is approximately 1340° to 1360° F. Forg- 
ings annealed at temperatures slightly above these show perfect 
annealing. If the annealing temperature should be raised, it is at 
the expense of ductility, and the fracture becomes coarsely crystalline 



CHROMIUM NICKEL STEELS 



359 



and shows " fire." The physical properties thus obtained are 
shown in the following table: 



Treatment. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elonga- 
tion 
Per Cent, 
in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


As forged 

Annealed at 1360°-1380° 

Annealed at 1550°-1575° 


86,500 
75,250 
74,300 

77,750 


45,000 
35,500 
35,000 
42,000 


21 
33 
30 
17 


34.6 
57.9 
50.1 
26.0 



Unless previously normalized (as by air cooling from a temper- 
ature such as used in the high-temperature anneal above), or double 
quenched as outlined in the treatments previously given, the harden- 
ing of sections of say 3 ins. diameter or more demands the use of 
a temperature some 200° over the critical range to bring out 
the full effects of this combination of chromium and nickel. This 
is true of both oil and water quenching, but more noticeably so 
in the case of oil baths. Parenthetically, it is interesting to compare 
such treatment with that which has been previously described under 
nickel steels; the technical reasons will then be clear. A hardening 
temperature of 1550° to 1580° F. is required if the steel has not been 
previously treated; take for example the following tests on an 8-in. 
round bar: 



Quenched in 


Drawn 


Tensile 


Elastic 


Elongation, 


Reduction 


Oil from 


at Deg. 


Strength, 


Limit, 


Per Cent. 


of Area, 


Deg. F. 


F. 


Lbs. per Sq. In. 


Lbs. per Sq. In. 


in 2 Ins. 


Per Cent. 


1450 


900 


79,750 


46,750 


28.0 


58.9 


1500 


900 


88,300 


53,000 


23.5 


55.4 


1580 


1050 


99,000 


71,500 


23.5 


61.9 


1580 


1050 


92,200 


63,600 


27.0 


62.4 


1580 


1050 


95,880 


70,700 


24.0 


62.0 



It will be noticed that the quenching heat of 1580° F. not only gives 
higher tensile strength, elastic limit and ductility, but also permits 
of a drawing temperature some 150° higher. Microscopically the 
structure obtained by the high-quenching temperature is excellent, 
as is shown in Fig. 241. The structure of the same piece after forging, 
and before treatment, is shown in Fig. 242. 

Steel with approximately 0.50 per cent, chromium, 1.50 per cent. 
nickel and about 0.40 carbon may be readily heat treated to fulfill 



360 



STEEL AND ITS HEAT TREATMENT 




Fig. 241.— Chromium Nickel Steel Axle Oil Quenched from 1580° F., Drawn at 
1050° F. X100. (BuUens.) 




Fig. 242.— Chromium Nickel Steel Axle as Forged. X100. (BuUens.) 



CHROMIUM NICKEL STEELS 



361 



the specification, in large sections up to 12 ins. diameter, and with 
proportionally higher tensile results in small sections, of: 

Tensile strength, lbs. per sq. in . . . .'. 90,000 

Elastic limit, lbs. per sq. in 60,000 

Elongation, per cent, in 2 ins 22 

Reduction of area, per cent 50 

The following equations connecting maximum strength, Brin- 
ell hardness number and scleroscope hardness number have been 
computed x from several hundred tests made with low chromium 
nickel steel (1.5 per cent, nickel and 0.5 per cent, chrome) of different 
carbon content and heat treated to bring out all possible physical 
properties : 

(1) M = 0.68 £-22. 

(2) Af = 3.7 S-l. 

(3) 5 = 5.4 £+33. 

M= maximum strength in units of 1000 lbs. per sq. in. 
B = the Brinell hardness number. 
$ = the scleroscope hardness number. 

The maximum strength corresponding to different Brinell val- 
ues as determined by equation (1) for these steels is as follows: 



Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


100 
150 
200 
250 
300 


46,000 

80,000 

114,000 

148,000 

182,000 


350 

400 
450 
500 
550 


216,000 
250,000 
284,000 
318,000 
352,000 



The maximum strength corresponding to different scleroscope 
values as determined by equation (2), and the corresponding Brin- 
ell numbers as determined by equation (3), for these steels, are as 
follows: 

1 R. R. Abbott, A. S. T. M., Vol. XV, Part II, 1915, p. 43 et seq. 



362 



STEEL AND ITS HEAT TREATMENT 



Scleroscope. 


Maximum Strength, 
Lbs. per Sq. In. 


- 
Brinell. 


20 
30 
40 
50 
60 
70 
80 
90 
100 


73,000 
110,000 
147,000 
184,000 
221,000 
258,000 
295,000 
332,000 
369,000 


141 
195 
249 
303 
357 
411 
465 
519 
573 



HIGH CHROMIUM NICKEL STEELS 

Chromium nickel steels containing approximately 3.5 per cent, 
nickel and 1.5 per cent, chromium comprise a type of steel with dis- 
tinctive physical characteristics, but which obviously are not shown 
by the results of ordinary pull test values when taken in comparison 
with the low chromium nickel steels. The following figures, giving the 
ordinary physical properties, illustrate the latter point. Dependent 
upon the section, treatment, and carbon content (0.2 to 0.5 per 
cent.), they may be given as follows: 



Composition. 


Tensile Strength. 


Elastic Limit. 


Elongation. 


Reducti on of Area. 


3.5 Nickel 
1 . 5 Chrome 

1 . 5 Nickel 
0.5 Chrome 


85,000 to 
275,000 

80,000 to 
264,000 


55,000 to 
265,000 

56,000 to 
240,000 


26 to 10 
30 to 8 


65 to .35 
70 to 27 . 5 



It is evident, since the above results show but little difference, 
that the superiority of the high chromium nickel steel does not appear 
in the static properties. On the other hand, there is a tremendous 
difference between the two types (in favor of the higher alloy) in 
the dynamic and endurance strength, such as freedom from brittle- 
ness and resistance to shock. This is illustrated by certain specific 
uses, as examples, to which these steels are put and 'which demand 
the highest attainable combination of dynamic strength, resistance 
to shock, and high static strength. Thus with about 0.2 to 0.3 
per cent, carbon these steels are used in protective deck plate, 
requiring that peculiar combination of properties which comprise 
ballistic strength; with a slightly higher carbon content, and cer- 
tain other modifications, we have a typical Krupp armor plate; and 



CHROMIUM NICKEL STEELS 



363 



with 0.45 to 0.50 per cent, carbon these steels are used in high-duty 
gears, and in which it is possible to hammer one tooth against its 
neighbor without breaking it off. 

Or, as it has been expressed in every-day terms, the effect of the 
larger amounts of alloys in suitable combination is like a comparison 
between a trained athlete and the amateur. Each man may be 
able to lift a maximum weight of say 200 lbs. But when it comes 
to repeating that same feat a number of times in succession, the 
trained man, with his developed powers of endurance, will win 
every time. And thus it is with the high alloy steel. 

Typical results for a steel of this type are given in the chart in 
Fig. 243. 

The following equations connecting maximum strength, Brinell 
hardness number and scleroscope hardness number have been com- 
puted ] from several hundred tests made with high chromium nickel 
steel (3.5 per cent, nickel and 1 per cent, chromium) of L different 
carbon content and heat treated to bring out all possible physical 
properties : 

(1) M=0.71 5-33. 

(2) M = 3.7 S-3 

(3) 5=4.8 £+58. 

M = maximum strength in units of 1000 lbs. per sq. in. 
B = the Brinell hardness number. 
S = the scleroscope hardness number. 

The maximum strength corresponding to different Brinell val- 
ues as determined by equation (1) for these steels is as follows: 



Brinell . 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


100 
150 
200 
250 
300 


38,000 

73,000 

109,000 

144,000 

180,000 


350 
400 
450 
500 
550 


215,000 
251,000 
286,000 
322,000 
357,000 



The maximum strength corresponding to different scleroscope 
values as determined by equation (2), and the corresponding Brin- 



!R. R. Abbott, A. S. T. M., Vol. XV, Part II, 1915, p. 43 et seq. 



364. 



STEEL AND ITS HEAT TREATMENT 





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Chemical Analysis: Critical Ranges: Size of Section: Treatment: 
C. 0.40 Aol, 2, 3 1305° IJnoh round Previously 
Mn. 0.40 Ar 1200° double quenched 
P. low to 000° In Oil and annealed. 
S. low Quenched in 
Si. 0.16 Oil from 1400 . 
Cr. 1.50 "id drawn 
Nl, 3.50 aB- given. 


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CHROMUM NICKEL STEELS 



365 



ell numbers as determined by equation (3), for these steels, are as 
follows : 



Scleroscope. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


20 
30 
40 
50 
60 
70 
80 
90 
100 


71,000 
108,000 
145,000 
182,000 
219,000 
256,000 
293,000 
330,000 
367,000 


154 
202 
250 
298 
346 
394 
442 
490 
538 



INTERMEDIATE TYPES OF CHROMIUM NICKEL STEELS 

Between the high and low composition types of chromium nickel 
steels previously given there are a great variety of combinations of 
the two alloying elements. Thus, in the chart in Fig. 244, we have 
the results obtained from the heat treatment of 1-in. rounds with 
1.0 per cent, chromium and 1.75 per cent, nickel. Other results, 
from similar compositions, taken from representative practice in the 
automobile world are given as follows : 

Carbon 0.26 to 0.35. 



Treatment. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic 

Limit, 

Lbs. per Sq. In. 


Elongation, 
Per Cent, 
in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


Brinell 
Hardness. 


Hardened. . . . 
Toughened. . . 
Untreated. . . . 


197,000 
110,000 
106,000 


135,000 
90,000 
70,000 


9 
25 
18 


37 
55 
45 


460 to 480 
235 to 250 



Carbon 0.46 to 0.55. 



Tempered . 
Toughened . 
Annealed. . 



305,000 

130,000 

95,000 



265,000 

114,000 

68,000 



5 
20 

26 



16 
60 
60 



480 to 525 
300 to 335 
180 to 200 



The effect of mass upon the latter type of chromium nickel steel 
is shown in Fig. 245. 

The chart in Fig. 246 gives the results of tests upon steel con- 
taining 0.75 per cent, chromium and 3.0 per cent, nickel, while Fig. 
247 illustrates a characteristic French steel containing 0.50 per cent, 
chromium and 2.50 per cent, nickel. 



366 



STEEL AND ITS HEAT TREATMENT 





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CHROMIUM NICKEL STEELS 



367 



Steels with 0.60 per cent, chromium, 3.5 per cent, nickel, and 
0.2 to 0.4 per cent, carbon, in medium-size forgings, may readily 
be treated to give a minimum of: 

Tensile strength, lbs. per sq. in 120,000 

Elastic limit, lbs. per sq. in 105,000 

Elongation, per cent, in 2 ins 20 

Illustrative of the relation of drawing temperatures to the carbon 




1 1% , 2 

Size in Inches 



Fig. 245. — Effect of Mass upon the Hardness, (Matthews & Stagg.) 



content for steels of this composition and with the same size of 
section, 1 to meet the above specification, the following may be of 
interest : 



Per Cent. Carbon. 


Drawing Temperature. 


0.24 
0.27 
0.36 


1150° F. 
1200° F. 
1240° F. 



1 Protective Deck Plate. 



368 



STEEL AND ITS HEAT TREATMENT 





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CHROMIUM NICKEL STEELS 



369 





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370 



STEEL AND ITS HEAT TREATMENT 



SPECIAL CHROMIUM NICKEL STEELS 

The following tests by Revillon on various combinations of car- 
bon, chromium and nickel in chromium nickel steels will be of interest : 

Analyses 



No. 


Carbon. 


Man- 
ganese. 


Phos- 
phorus. 


Sulphur. 


Silicon. 


Chromium. 


Nickel. 


1 


.22 


.54 


.009 


.044 


.36 


.35 


2.19 


2 


.25 


.25 


.027 


.043 


.08 


.48 


2.75 


3 


.425 


.27 


.006 


.042 


.20 


1.20 


2.86 


4 


.17 


.53 


.006 


.053 


.16 


.18 


3.47 


5 


.105 


.43 


.014 


.030 


.11 


.85 


4.38 


6 


.86 


.23 


.014 


.030 


.11 


.86 


.88 


7 


.25 


.52 


.006 


.053 


. 17 


1.28 


3.82 


8 


.31 


.70 


.014 


.021 


.17 


1.48 


2.75 


9 


.42 


.22 


.013 


.057 


.11 


.31 


4.09 


10 


.45 


.28' 


.014 


.030 


.11 


.58 


2.25 


11 


.52 


.27 


.006 


.030 


.39 


.43 


2.80 


12 


.77 


..32 


.014 


.030 


.11 


.19 


1.13 


13 


.10 


.35 


.003 


.035 


.31 


1.75 


5.36 


14 


.265 


.24 


.014 


.030 


1.27 


2.33 


4.40 


15 


.27 


.39 


.014 


.030 


.11 


.85 


4.90 


16 


.36 


.37 


.006 


.053 


.23 


1.15 


4.20 


17 


.39 


.68 


.018 


.021 


.35 


.78 


5.19 



Critical Points 



No. 


Degrees Fahr. 


No. 


Degrees Fahr. 




Ac. 


Ar. 


Ac. 


Ar. 


1 

2 
3 
4 
5 
6 
7 
8 
9 


1472 
1463 
1508 
1454 
1427 
1418 
1355 
1454 
1400 


1256 

1274 

1265 

1274 

1139 

1301 

941 

617 

788 


10 
11 
12 
13 
14 
15 
16 
17 


1418 
1472 
1508 
1400 
1400 
1490 
1418 
1436 


1229 
1283 
1301 
860 
986 
986 
770 
482 



CHROMIUM NICKEL STEELS 



371 



Annealed 





Annealing 


Tensile 


Elastic 


Elon- 


Reduc- 


Guillery 
Shock 

Test. 


Brinell 




Temper- 


Strength, 


Limit, 


gation, 


tion 


Hard- 




ature, 


Lbs. per 


Lbs. per 


in 2 Ins. 


of Area, 


ness 




Deg. F. 


Sq. In. 


Sq. In. 


Per Cent. 


Per Cent. 


Number. 


1 


1472 


80,690 


56,320 


26 


64.9 


133.8 


153 


2 


1472 


87,690 


61,160 


23 


55.7 


65 


170 


3 


1292 


105,400 


73,670 


22 


63.2 


112.1 


197 


4 


1652 


87,760 


50,200 


21.5 


53 


36.1 


168 


5 


1472 


90,600 


60,160 


20 


60.5 


115.7 


179 


6 


1382 


137,960 


72,960 


11 


23.8 


21.7 


210 


7 


1292 


114,650 


64,570 


17.5 


50.5 






8 


1112 


134,410 


126,440 


14.5 


59.2 


54.2 


250 


9 


1112 


115,930 


99,560 


18 


65.8 


101.2 


217 


10 


1382 


122,320 


71,120 


13 


45.5 


43.4 


220 


11 


1382 


135,830 


81,780 


14 


49.8 


54.2 


251 


12 


1112 


119,610 


83,910 


9.5 


46.7 


39.8 


273 


13 


1112 


163,850 


142,940 


13.5 


58.4 


137.4 


178 


14 


1652 


123,030 


75,100 


5.5 


9.8 


68.6 


232 


15 


1382 


142,510 


93,870 


6.5 


28 


39.8 


288 


16 


1112 


128,290 


119,610 


17 


62.4 


50.6 


225 


17 


1112 


147,210 


91,600 


14.5 


52.8 


47 


268 



Note: It will be noticed that in a number of instances the temperature used in the 
above annealing is under the Ac point, which will explain the high tensile results obtained. 
Such cases do not represent full annealing. 



Heat Treated 



No. 


Quenching 
Bath and 

Temper- 
ature, 

Deg. F. 


Draw- 
ing 
Tem- 
pera- 
ture 

Deg. F. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation 
in 2 Ins. 
Per 
Cent. 


Reduc- 
tion of 

Area, 
Per 

Cent. 


Guil- 
lery 
Shock 
Test. 


Brinell 
Hard- 
ness 
Num- 
ber. 


1 


Water, 


1382 




204,500 


180,510 


10 


44.3 


68.6 


370 


2 


Oil, 


1472 




225,010 


189,170 


7 


19.3 


47 


418 


3 


Oil, 


1472 


572 


264,550 


214,820 


6.3 


42.7 


54.2 


412 


4 


on, 


1562 




197,700 


173,520 


5 


15.4 


61.5 


328 


5 


Water, 


1382 




201,970 


173,520 


10 


54 


72.3 


295 


6 


Oil, 


1382 


932 


230,410 


221,880 


1.5 


4.3 


36.1 


388 


7 


Oil, 


1562 




208,370 


183,350 


9.5 


51 


47 


343 


8 


Oil, 


1472 




292,280 


289,440 


9.5 


36.3 


57.8 


425 


9 


Air, 


1472 




221,020 


189,870 


8 


41 


54.2 


396 


10 


Water, 


1472 


932 


190,580 


173,520 


6.5 


47.8 


79.5 


301 


11 


Oil, 


1472 


572 


215,530 


202,250 


7 


42 


43.4 


395 


12 


Oil, 


1472 


932 


210,500 


181,930 


6 


14.1 


32.5 


425 


13 


Water, 


1472 




188,020 


168,970 


10 


56 


72.3 


286 


14 


Water, 


1562 




183,190 


160,010 


10 


52.5 


57.8 


298 


15 


Water, 


1382 


932 


187,640 


157,730 


7 


45.7 


72.3 


300 


16 


Air, 


1562 




235,390 


225,860 


9 


24.5 


54.2 


402 


17 


Air, 


1472 












32.5 


512 



372 STEEL AND ITS HEAT TREATMENT 

CHROMIUM NICKEL STEEL IN AUTOMOBILE CONSTRUCTION 

0.25 Carbon and under 

Principally for case-hardening purposes, such as bevel driving 
and transmission systems, steering-wheel pivot pins, cam rollers, 
push rods, and similar parts which must not only have a hard 
exterior surface, but possess strength as well. 

0.25 to 0.35 Carbon 

Axles. — Steering knuckles, bolts, pinions, steering pivots, 
spindles, driving shafts, etc., gears with light case, drawn. Gears 
hardened, but not drawn. 

This grade of chrome nickel steel forges and machines well, 
and responds to heat treatment in matter of strength as well as of 
toughness. 

0.35 to 0.45 Carbon 

Crankshafts. — Countershafts, propeller shafts, live axles, 
diving shafts. 

This grade possesses under suitable heat treatment a high 
degree of strength and considerable toughness. Its fatigue-resisting 
(endurance) properties are extremely high. 

0.45 to 0.55 Carbon 

Tempered Gears.— This grade probably gives the greatest 
possible hardness with the least possible brittleness (in combination) 
of any steel for transmission purposes. 

MAYARI CHROMIUM NICKEL STEEL 

Mayari steel is a " natural alloy " steel containing from .20 per 
cent, to .70 per cent, chromium and 1.00 per cent, to 1.50 per cent, 
nickel. It is made from a low-phosphorus Cuban ore containing the 
alloying elements chromium and nickel. In the blast furnace the 
chromium and nickel in the ore are reduced, forming a natural con- 
stituent of the iron. By means of the duplex process — Bessemer 
converter and open hearth — the iron is then made into steel, the 
chromium and nickel pass into the steel, forming a natural alloy, 
with no other addition of these elements in the furnace or ladle being 
necessary. Mayari steel has given excellent satisfaction in a large 
number of cases, although it undoubtedly is not equal to synthetic 
chromium nickel steel where the highest quality chromium nickel 
steel is required. 



CHROMIUM NICKEL STEELS 



373 



In the natural or forged condition Mayari steel has from 8000 
to 10,000 lbs. per square inch higher tensile strength and elastic 
limit than a carbon steel of the same carbon content. Like all 
alloy steels, it welds with more or less difficulty by the ordinary 
methods, and would not be recommended for purposes where a 
welded part is subject to great strains. By careful work in a Thomp- 
son electric welding machine, excellent results are obtained, so that 
where this method is applicable Mayari steel may be welded satis- 
factorily. 

The physical properties of Mayari steel, heat treated, in | in. 
bars, are shown in Figs. 248, 249 and 250. The effect on the physical 
properties of variation in the size of the piece treated is indicated in 
the charts, Figs. 251 and 252, which show the properties of heat- 
treated rounds from 1 in. to 6 ins., and 7 in. to 4^ ins. diameter, 
respectively. All of the rounds on the same chart were from the 
same heat of steel. These were treated together at the same time in 
exactly the same manner. The first chart is 0.28 per cent, carbon, 
and the second 0.39 -per cent, carbon; both grades contained 0.45 
per cent, chromium with the usual nickel. On the bars over 2 ins. 
in diameter the tests were taken one-half the distance from the 
center to the outside, and on the smaller rounds they. were taken 
from the center. 

The following table 1 shows the approximate difference in draw- 
ing temperatures for Mayari steel of larger sizes than those given 
in the charts of Figs. 248 to 250. When it is desired to obtain the 
same elastic limit on a size larger than f-in. diameter, find the draw- 
ing temperature on the chart, then by making the allowance given 
in the table below for the size desired, the proper temperature for 
this elastic limit will be determined. The other properties will 
vary from those on the chart by the percentage shown in the table. 





Change in 

Drawing 

Temperature. 


Physical Properties; Per Cent, of that given on Charts for 
J-in. Rounds. 


Diameter. 


Tensile 
Strength, 
Per Cent. 


Elastic 

Limit, 

Per Cent. 


Elongation, 
Per Cent. 


Reduction of 

Area, 

Per Cent. 


1 in - 
2§ ins. 
3| ins. 
4J ins. 



- 90° F. 
-135° F. 
-235° F. 


100 
102 
110 
122 


100 
100 
100 
100 


100 
90 

87 
80 


100 

96 
85 
83 



J Penna. Steel Co. 



374 



STEEL AND ITS HEAT TREATMENT 





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CHROMIUM NICKEL STEELS 



371 





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376 



STEEL AND ITS HEAT TREATMENT 





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CHROMIUM NICKEL STEELS 



377 



_• 290 000 
M 2S0 000 
£270 000 

to 2t;o ooo 

^ 27)0 000 
p. 240 000 
_; 230 000 
h 220 000 
•J 210 000 
-3 200 000 

g 190 ooo 

£ ISO 000 
m 170 000 

3 ico ooo 

"S 150 000 

£ no ooo 

M 130 000 
"3 120 000 
I 110 000 
J 100 000 
tj, 90 000 
C 80 000 
£ 70 000 

3a 60000 

(D 50 000 
'•- 40 000 
C 30 000 
& 20 000 
10 000 




=5S^Hrtioii 



l£rea 



2^si| 8 { 



^J^o^. 




a 

105$ w 



555^2 
505^ 

45* £ 




3554? 



3 

25* « 



Fig. 251. — Effect of Size on Physical Properties of Mayari Steel, 0.30 Carbon. 
Same Analysis and Same Treatment, (Penna. Steel Co.) 



u GO 000 

3a 50000 

0> 40 000 

■55 30 000 

g 20 0Q0 

H 10 000 



p 280 000 
M 270 000 
& 200 000 
m 250 000 
£ 240 000 

P. 230 000(-Reduction-of-Area 

DO 220 000 

ri 210 000 
H 200 000 
,-4 190 000 
g 180 000 
•3 170 000 
*2 160 000 
■2 150 000 
g 140 000 
« 130 000 
"J 120 000 

"2 110 000 
a 100 000 
■S 90 000 



-Tensile Str 



90 UOU rq ~. — — 

80 000 t-isss-afflSJ" !': 

70 000 



% round 




2Yi louiul 



3^ lound 



G5# 
60^ 

55$ 
50*^ 
45* ! 
40* ; 

359s ; 

305f ' 

25^ ; 

205*'. 
15 * 
10^ f 
556 



4^ luund 



Fig. 252.— Effect of Size on Physical Properties of Mayari Steel, 0.40 Carbon. 
Same Analysis and Treatment, (Penna. Steel Co.) 



CHAPTER XVI 
VANADIUM STEELS 

The principal effect of vanadium additions to steel is its 
effect upon the physical characteristics of the steel. Like most 
alloys, vanadium tends to give a finer and denser structure 
than that ordinarily obtained in straight carbon steels. In true 
vanadium steels, i.e., steels in which vanadium is present in definite 
commercial quantities, the general action of vanadium is similar 
to that of many of the alloys previously discussed, but it also 
presents other interesting phenomena. Vanadium, in the regular 
steels containing about 0.12 to 0.20 per cent, vanadium, is prob- 
ably present in both- the ferrite (similar to nickel) and also as a 
double carbide in the cementite (similar to chromium). In support 
of the first statement that vanadium is in solid solution in the ferrite 
are the results of many tests which appear to show that the duc- 
tility is higher in these commercial vanadium steels than in corre- 
sponding steels which do not contain vanadium. This is based on 
the assumption, generally accepted, that the nature of the ferrite 
element is indicative, to a large degree, of the ductility of the steel. 

The proof direct that vanadium forms a double carbide is illus- 
trated by steels with higher percentages of contained vanadium. 
Thus steels containing 0.2 per cent, carbon and up to 0.7 per cent, 
vanadium, or 0.8 per cent, carbon and 0.5 per cent, vanadium, are 
normally pearlitic; but any increase in the vanadium content over 
these limits will produce a characteristic double carbide component. 

From these limiting ratios of carbon and vanadium it is evident 
that vanadium has a powerful influence upon the transformation 
ranges — more so, indeed, than any of the common alloying elements. 
This also goes to show the reason why only small quantities — 
0.25 per cent, vanadium or under — are necessary to produce a 
noticeable effect. 

As a general proposition, any alloy which tends to form a cemen- 
titic compound in steel also has the tendency to require a higher 
temperature for quenching in order to bring the steel as a whole 
into a state of equalization. This was found to be true in the case 

378 



VANADIUM STEELS 379 

of chromium, and it is also true of vanadium steels. A study of the 
heat-treatment data subsequently given will show that vanadium 
alloy steels give the best results with an apparently abnormally high 
quenching temperature, or at about 1560° to 1600° F. for the medium 
carbon grades. This point is also illustrated by the photomicro- 
graph in Fig. 253, which illustrates the structure of a rolled plate of 
" Type A " chromium vanadium steel oil treated at the temper- 
atures best suited for a chromium nickel steel of the same carbon 
and manganese content. From the structure thus shown it is 
evident that the steel as a whole has not been equalized at the tempera- 




Fig. 253. — Chromium Vanadium Steel, Type A, Oil Treated at the Same Tem- 
peratures Used for a Corresponding Chromium Nickel Steel. X 60. (Bullens.) 

ture for hardening (1500° F.) which was used, since the ferrite (white) 
is still segregated and tends to follow the lamellar structure of the 
original steel. 

On the other hand, if we follow out the characteristics peculiar 
to most alloy steels of a carbide nature, we would expect that the 
vanadium steels would be inherently more sensitive to prolonged 
heating or rapid cooling. Now while it is true that steels containing 
vanadium will give a greater depth of hardness upon suitable quench- 
ing than will some steels of a ferritic nature (such as nickel steels), 
it does not appear to be true that vanadium abnormally increases 
the sensitiveness of the steel to prolonged heating. This appears 
to be one of the anomalies of vanadium steels. 



380 



STEEL AND ITS HEAT TREATMENT 



Viewed from the standpoint of physical test values, vanadium 
requires the presence of another alloy as an " intensifier," in order 
that the full effect and influence of the vanadium additions may be 
felt. Just as chromium greatly intensifies the influence of nickel in 
steel, so chromium also seems to bring out the latent capabilities of 
vanadium, but to an even greater extent. Thus the majority of 
the vanadium steels now in commercial use are of the chromium 
vanadium type. 

The predominant note which is always sounded when speaking 
or writing about chromium vanadium or vanadium steels is that 
of increased dynamic strength. There is little doubt that vana- 
dium greatly increases the dynamic strength in comparison with 
that of a corresponding straight carbon steel. Upon the relative 
merits, as regards dynamic strength, of chromium vanadium and 
chromium nickel steels, we have commented under the latter steels. 
Extensive tests made to determine dynamic strength have led to 
varying results, and it is deemed best to leave the subject with the 
warning that each steel has its specific field of usefulness and most 
advantageous application. 

The following equations connecting maximum strength, Brinell 
hardness number and scleroscope hardness number have been 
computed T from several hundred tests made with chromium vana- 
dium of different carbon content and heat treated to bring out all 
possible physical properties: 

(1) M = 0.71 5-29. (2) M=4.2 S-21. (3) 5 = 5.5 £ + 27. 
M = maximum strength in units of 1000 lbs. per sq. in. 
5 = the Brinell hardness number. 
S = the scleroscope hardness number. 

The maximum strength corresponding to different Brinell val- 
ues as determined by equation (1) for these steels is as follows: 



Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


Brinell. 


Maximum Strength, 
Lbs. per Sq. In. 


100 
150 
200 
250 
300 


42,000 

77,000 

113,000 

148,000 

184,000 


350 
400 
450 
500 
550 


219,000 
255,000 
290,000 
326,000 
361,000 



1 R. R. Abbott, A. S. T. M., Vol. XV, Part II, 1915, p. 43 et seq. 



VANADIUM STEELS 



381 



The maximum strength corresponding to different scleroscope 
values as determined by equation (2), and the corresponding Brin- 
ell numbers as determined by equation (3), for these steels, are as 
follows : 



Scleroscope. 


Maximum Strength, 
Lbs. per Sq. In. 


— 

Brinell. 


20 
30 
40 
50 
60 
70 
80 
90 
100 


63,000 
105,000 
147,000 
189,000 
231,000 
273,000 
315,000 
357,000 
399,000 


137 
192 
247 
302 
357 
412 
467 
522 
577 



Static test results 1 upon various 
follow: 



types " of vanadium steels 



In part by the American Vanadium Co., Pittsburgh, Pa. 



382 



STEEL AND ITS HEAT TREATMENT 



Type " A " Chromium-vanadium Steel 

Tests from Small Sections 

Carbon 26% Manganese 

Chromium 92% Silicon 

Vanadium 20% 



• 48% 
.06% 



Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 Ins.,%. 


Reduction 
of Area, %. 


As rolled 


132,000 


110,000 


19.0 


51.5 


Annealed 1475° F 


83,700 


61,000 


34.8 


66.4 


Oil tempered: 










1650°-1155°F 


133,000 


99,020 


30.0 


69.9 


1650-1110 


137,000 


112,000 


20.0 


61.0 


1650 -1020 


141,500 


123,000 


18.0 


63.5 


1650 - 930 


162,700 


146,250 


15.0 


57.0 


1650 - 840 


177,500 


151,500 


14.0 


53.0 


1650 - 750 


183,500 


155,000 


13.0 


51.0 


1560-1155 


131,000 


100,000 


28.0 


67.0 


1560-1110 


133,000 


108,400 


17.5 


65.4 


1560 -1020 


137,500 


112,750 


21.0 


64.5 


1560 - 930 


156,800 


138,440 


16.5 


59.8 


1560 - 840 


171,100 


147,150 


15.0 


61.0 


1560 - 750 


173,900 


149,800 


13.0 


57.0 


Water tempered: 










1650°-1155°F 


156,000 


133,000 


18.0 


62.5 


1650-1110 


160,900 


149,700 


16.0 


60.4 


1650 -1020 


167,800 


151,000 


12.0 


53.6 


1650 - 930 


183,200 


166,800 


12.5 


56.5 


1650 - 840 


204,800 


176,200 


12.5 


54.5 


1560-1155 


153,050 


136,600 


27.0 


60.0 


1560-1110 


156,500 


146,300 


17.0 


61.0 


156C -1020 


166,800 


149,100 


14.0 


58.9 


1560 - 930 


176,950 


165,000 


14.0 


59.0 


1560 - 840 


201,800 


172,800 


12.5 


54.5 



Tests from Medium Sections 



Carbon . . . 
Chromium , 



23% Manganese 58% 

82% Silicon 105% 

Vanadium 17% 



Stock. 


Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 Ins., %. 


Reduction 
of Area, %. 




Oil tempered: 










2J-in. 


1650°-1050°F 


125,730 


108,950 


19.0 


60.4 


2^-in. 


1650 -1050 


124,160 


106,000 


20.0 


60.6 


2f-in. 


1650-1050 


122,740 


104,750 


19.5 


57.0 


2|-in. 


1650-1050 


126,700 


111,500 


17.0 


53.0 


2|-in. 


1650-1050 


121,080 


106,500 


18.0 


60.7 


2|-in. 


1650 -1050 .. . 


124,130 


107,000 


18.5 


61.1 



VANADIUM STEELS 



383 



Test from Q-in. Tender Axle 

Carbon 29% Manganese 28% 

Chromium 1 .00% Silicon 06% 

Vanadium 20% 



Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 Ins.,%. 


Reduction 
of Area, %. 


Water tempered: 
1690°-1155°F 


115,000 


90,000 


21.0 


55.0 



Tests from Locomotive Driving Axles, 10 Ins. Diameter 
Average Test of 287 Heat-treated Axles 

Carbon 35% Manganese 50% 

Chromium 90% Vanadium 22% 

Elastic limit, pounds per square inch 81,600 

Tensile strength, pounds per square inch 108,890 

Elongation in 2 ins., per cent 21 . 75 

Reduction of area, per cent 58 . 75 

Type " D " Chromium Vanadium Steel 
Tests on Small Sections 

Carbon 50% Manganese 92% 

Chromium 1 .02% Silicon 065% 

Vanadium 20% 



Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 Ins.,%. 


Reduction 
of Area, %. 


Brinell 

Hardness 

No. 


As rolled 


153,350 
103,440 


124,450 
63,660 


12.5 

25.8 


37.0 
61.5 


286 


Annealed 1475° F 


187 


Oil tempered: 












1650°-1110° F 


186,800 


170,000 


15.5 


45.2 


340 


1650 -1020 


201,150 


186,100 


13.0 


45.5 


364 


1650 - 930 


209,800 


192,200 


12.5 


42.5 


364 


1650 - 840 


227,040 


217,360 


10.0 


35.5 


402 


1650 - 750 


264,500 


239,700 


6.5 


17.0 


444 


1600 -1110 


186,100 


161,200 


13.5 


45.5 


340 


1600-1020 


205,500 


187,000 


12.0 


45.0 


340 


1600 - 930 


214,050 


203,600 


11.5 


43.0 


380 


1600 - 840 


237,500 


221,000 


10.0 


29.5 


418 


1560 -1020 


197,100 


187,100 


12.5 


45.0 


340 


1560 - 930 


214,270 


201,400 


11.5 


36.0 


418 


1560 - 840 


234,150 


215,850 


9.0 


28.5 


418 


1560 - 750 


261,850 


240,000 


7.0 


22.0 


418 


1520 -1020 


183,500 


177,250 


14.5 


47.5 


340 


1520 - 930 


215,450 


193,100 


12.0 


41.5 


387 


1520 - 840 


237,750 


213,400 


10.0 


35.5 


387 


1520 - 750 


260,500 


240,000 


8.0 


24.0 


444 



384 



STEEL AND ITS HEAT TREATMENT 



600 



a 400 



300 



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Effect ot Mass 
C. 0.49 S. 0.01 
Si. 0.10 Cr. 1.18 
Mn. 0.74 V. 0.18 
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2£ 1 1M 1/2 1& 2 2J4 2i/ 2 2% 3 3)4 
Size in Inches 



Fig. 254. — Effect of Mass upon the Hardness of Chromium Vanadium Steel. 
(Matthews and Stagg.) 



The effect of mass upon the hardness of steel of this type is 
shown in Fig. 254. 1 



Type " G " Chrome Vanadium Steel 

Carbon 60% Manganese 54% 

Chromium 88% Silicon 175% 

Vanadium 19% 



Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 ins.,%. 


Reduction 
of Area, %. 


Brinell 

Hardness 

No. 


Oil tempered: 
1650°-1110° F 
1650 - 930 
1650 - 750 


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240,400 
273,000 


179,300 
220,000 
248,660 


13.0 

10.0 

8.0 


37.0 
28.3 
27.3 


402 
477 
532 



1 From Matthews and Stagg, " Factors in Hardening Tool Steel." 



VANADIUM STEELS 

Nickel Vanadium Steel 



Carbon . 
Nickel. . 



29% Magnanese... 

3.41% Silicon 

Vanadium 20% 



385 



• 45% 
.090% 



Treatment. 


Tensile 
Strength. 


Elastic 
Limit. 


Elongation 
in 2 ins.,%. 


Reduction 
of Area, %. 


Annealed 800° C... . .. 


107,300 


73,000 


23.5 


55.5 


Oil tempered: 










1600°-1160°F 


148,300 


126,250 


18.0 


58.0 


1600-1110 


150,000 


128,500 


17.5 


57.4 


1600 -1020 


151,500 


132,500 


16.0 


56.9 


1600 - 930 


162,000 


144,200 


14.5 


52.6 


1600 - 840 


178,200 


157,210 


13.0 


52.7 


1600 - 750 


193,200 


163,000 


12.0 


50.2 


1520-1160 


137,700 


123,000 


16.0 


59.0 


1520-1110 


140,700 


125,500 


17.5 


54.2 


1520 -1020 


148,100 


126,800 


16.5 


55.0 


1520 - 930 


154,900 


135,000 


15.5 


57.2 


1520 - 840 


165,800 


146,500 


14.0 


55.2 


1520 - 750 


181,000 


162,800 


14.0 


53.5 


Water tempered: 










1600°-1160°F 


148,000 


126,700 


18.5 


58.1 


1600-1110 


153,800 


133,100 


15.0 


58.8 


1600 -1020 


156,300 


136,500 


14.0 


54.5 


1600 - 930 


161,200 


146,700 


14.5 


56.4 


1600 - 840 


186,400 


173,300 


13.0 


52.7 


1600 - 750 


195,200 


176,580 


12.0 


52.2 


1520 -1160 


139,800 


128,570 


18.5 


59.7 


1520-1110 


146,000 


132,250 


14.0 


57.5 


1520 -1020 


154,600 


133,900 


15.5 


56.3 


1520 - 930 


160,400 


144,600 


15.0 


51.7 


1520 - 840 


184,500 


176,750 


13.0 


53.0 


1520 - 750 


199,300 


182,700 


12.0 


50.0 



386 



STEEL AND ITS HEAT TREATMENT 





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CHAPTER XVII 

MANGANESE, SILICON, TUNGSTEN, AND MOLYBDENUM 

STEELS 

MANGANESE STEELS 

The term " manganese steel," by commercial usage, generally 
refers to steels with that high percentage of manganese which will 
cause the metal to become austenitic under the conditions of ordinary 
cooling or suitable heat treatment. But before proceeding to a 
discussion of such steels, it is desirable to amplify the remarks we 
have previously made upon the subject of pearlitic manganese steels. 

PEARLITIC MANGANESE STEELS 

Since manganese forms with cementite a double carbide in steel, 
the effect is to increase the tensile strength of the metal. In untreated 
steel this beneficial effect, according to Campbell (see page 3), 
is dependent upon the carbon content. In treated steel each 0.1 
per cent, of manganese — up to about 2 per cent. — will increase the 
tensile strength by about 1500 to 2000 lbs. per sq. in. The effect 
upon the Brinell hardness (and, therefore, the tensile strength) of 
increasing amounts of manganese, with a constant carbon content, 
is shown in Fig. 255, plotted from data obtained from quenching and 
drawing samples \ in.X| in. Xf in. in size. 

A comparison l of the effect of heat treatment upon the physical 
properties of standard test bars of the same size of a manganese 
steel (0.34 per cent, carbon, 1.61 per cent, manganese), a nickel 
steel (0.34 per cent, carbon, 0.55 per cent, manganese, 3.17 per cent, 
nickel), and a straight carbon steel (0.34 per cent, carbon, 0.54 per 
cent, manganese), with the same carbon content, is shown in the 
charts of Figs. 256 and 257. From these charts it is evident that 
the tensile strength and elastic limit of the manganese and nickel 
steels are practically alike, so that it can be concluded that for a 
heat treated 1.5 per cent, manganese steel the manganese in excess 
of that contained in a nickel steel of a corresponding carbon content 

1 R. R. Abbott, A. S. M. E., June, 1915. 
387 



388 



STEEL AND ITS HEAT TREATMENT 



(0.34 per cent.) exerts a strengthening effect equivalent to about 
three times the same amount of nickel. 

These charts would also indicate that the manganese steel in the 



1000 



800 



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400 



200 





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C 0.75 " 0.65 
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200 



400 600 800 1000 

Drawing Temperature, Deg. Fahr- 



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1400 



Fig. 255. — Effect of Manganese upon the Brinell Hardness. 



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Temperature of Draw, °Fahr. 



Fig. 256. — Effect of Heat Treatment on Tensile Strength (upper curves) and 
Elastic Limit (lower curves). (Abbott.) 



MANGANESE, SILICON AND OTHER ALLOY STEELS 389 

heat-treated condition does not compare so favorably in ductility — 
as represented by the reduction of area and elongation — with the 



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Fig. 258. — Effect of Over- and Under-heating on Tensile Strength and Elastic 

Limit. (Abbott.) 



carbon and nickel steels. This is not supported by the results of 
hundreds of tests made upon heat-treated rifle barrels analyzing 



390 



STEEL AND ITS HEAT TREATMENT 



0.45 to 0.55 per cent, carbon with about 1.25 per cent, manganese; 
upon physical test these gave 80,000 to 90,000 lbs. per sq. in. tensile 
strength, 115,000 to 125,000 lbs. per sq. in. elastic limit, 22 to 24 
per cent, elongation and about 55 to 60 per cent, reduction of area. 
Nor do the results obtained from large sections of 0.40 per cent, car- 
bon, 1.0 per cent, manganese, steel appear to give less ductility than 
a 3.5 per cent, nickel steel of the same carbon content. In the 
annealed condition, however, the comparison of the three steels 
noted in the previous paragraph show that the supposedly deleterious 
effect of manganese upon the ductility is not evident, as is illus- 
trated by the following results from annealed pieces: 



Steel. 


Tensile 

Strength, 

Lbs. per 

Square Inch. 


Elastic 

Limit, 

Lbs. per 

Square Inch. 


Elongation, 

Per cent, in 

2 Inches. 


Reduction 
of Area, 
Per cent. 


Brinell 
Hardness 


Manganese 

Nickel 

Carbon 


87,850 
81,850 
67,250 


61,150 
55,000 
36,600 


29.9 
31.2 
32.0 


58.5 
59.0 
51.0 


150 
153 
120 







The results shown in the chart, 1 Fig. 258, indicate that there is 
very little difference between the manganese and nickel steels as 
far as over- and under-heating during the heating for quenching is 
concerned. The results shown were obtained from quench'.ng the 
test bars from the temperature indicated, followed by a draw-heat 
at 800° F. 

Practical experience confirms the fact that the period of satura- 
tion exerts a tremendous influence upon the results to be obtained 
from the heat treatment of pearlitic manganese steels (see page 203). 

The effect of manganese in normally pearlitic manganese steels 
upon the fragility of the steel is a more or less undetermined factor. 
Many persons have undoubtedly confused the subject of inherent 
brittleness or non-resistance to shocks with the results caused by the 
sensitiveness of these steels to certain heat-treatment methods. 
They have assumed, because a piece of steel with ,1 to 2 per cent, 
manganese may have cracked on drastic water quenching, that the 
steel was " brittle," when, as a matter of fact, this result was prob- 
ably due to reasons entirely apart from the dynamic strength of the 
metal. Thus it may be said that this situation has led to the belief 
that manganese contributed an embrittling effect to the steel. Even 

1 It. R. Abbott, A.S.M.E., June, 1915. 



MANGANESE, SILICON AND OTHER ALLOY STEELS 391 

assuming that such an influence may exist in the case of very high- 
carbon steels, it distinctly has not been proven to be true of the 
hypo-eutectoid steels. In fact, there is now considerable evidence 
which tends to show that the lower-carbon pearlitic manganese 
steels, when properly made and suitably heat treated, are not brittle; 
the manganese appears to make the steel more sensitive to ill-treat- 
ment. If the steel is made in small heats, has been thoroughly 
refined, and with the elimination of impurities to a minimum, a 
great deal may be accomplished with such steels. Certain manga- 
nese steels with 1.5 to 2 per cent, manganese and a considerable car- 
bon content, made in the electric furnace, have shown wonderful 
mechanical properties, and, in addition, will stand a tremendous 
amount of abuse in their thermal treatment without any great ill 
effects. 

In treating pearlitic manganese steels it should be remembered 
that each 0.1 per cent, manganese will lower the critical range on 
heating by about 5° to 6° F., so that lower temperatures may, and 
in most cases should, be used for their hardening or full annealing. 
In general, the effect of manganese on the critical ranges is about 
twice that of nickel. 

HIGH-MANGANESE STEELS 

In general, the requirements for producing a commercial mangan- 
ese steel necessitate a manganese content of about 6 or 8 per cent, 
to 20 per cent., in combination with the proper amount of carbon. 
Below the lower limits given, the steel, even by the most suitable 
treatment, may be characterized by the presence of weak and 
brittle martensite. The upper limits are determined by the cost 
of the manganese additions, and further, by the again predominating 
influence of the carbon content (when the manganese rises to around 
20 per cent.), which will make the steel stiff and brittle when cold. 
Most manganese steels will have about 11 or 12 per cent, manganese 
and about 1.0 to 1.2 per cent, carbon. 

Recent research work along the lines of determining the proper 
combination of carbon and manganese has greatly widened the 
commercial range for the manganese content, so that the more 
recent steels have the tendency toward a percentage of manganese 
lower than that originally thought necessary. Similarly, the field 
for the use of high manganese steels has also been considerably 
broadened. Above all, however, the peculiar merit of these steels 
lies in the resistance to abrasive wear, in combination with suffi- 



392 



STEEL AND ITS HEAT TREATMENT 



cient strength and ductility. In this regard, manganese steels 
appear to resist the abrasive wear characteristic of heavy impacts 
of hard substances better than that caused by the sliding attrition 
of hardened parts, or like that of an abrasive wheel. 

Aside from the dynamic strength, the selection of a manganese 
steel for any specific work depends upon the correlation of wear- 
ing qualities and static properties. In general, and in connection 
with a maximum wear resistance, it may be said that the most 
ductile steel which will give an elastic limit sufficiently high to avoid 
distortion in service will be best. And these, in turn, depend upon 
the proper combination of carbon and manganese. Thus a steel 
with 9 to 11 per cent, manganese and the proper amount of carbon 
will have a higher elastic limit than a steel with over 11 per cent, 
manganese. Again, steel with 11 per cent, manganese and 1.10 
per cent, carbon will have a higher elastic limit than a steel with 
15 per cent, manganese and 0.8 per cent, carbon. With high man- 
ganese and low carbon, steels quenched in water from 1830° F. will 
give a low elastic limit and a flow of metal which may prove excessive 
for many duties. A great deal also depends upon a suitable heat 
treatment of the steel. 

As might be expected from our knowledge of the influence of 
the rate of cooling upon the structure of high-alloy steels, the physical 
properties of these high-manganese steels are greatly modified by 
the method of casting, the size of the casting, and the mechanical 
elaboration. The first two factors in particular have a great influence 
upon the toughness of the metal. The average tests of commercial 
manganese steels with about 11 or 12 per cent, manganese and a 
little over 1.0 per cent, carbon will give approximately the following: 



Condition of the metal. 


Tensile Strength, 
Lbs. per Sq. In. 


Elastic Limit, 
Lbs. per Sq. In. 


Elongation, 
% in 2 Ins. 




82,000 

135,000-140,000 

142,000 


45,000 

60,000-70,000 
55,000 


30 


Rolled 


30-40 


Forged 


38 







The elastic limit of some sections as rolled may even go as high as 
75,000 lbs. per square inch; the proper heating and working of the 
metal plays a very important part in the results to be obtained on 
physical test. 

A common specification for manganese steel rails is as fol- 
lows: 



MANGANESE, SILICON AND OTHER ALLOY STEELS 393 

Chemical: 

Carbon, per cent . 95 to 1 . 15 

Manganese, per cent 10 to 13 

Silicon, per cent 0. 20 to 0.40 

Phosphorus, per cent under 0. 10 

Sulphur, per cent under . 06 

Physical : 

Tensile strength, lbs. per sq. in 100,000 

Elastic limit, lbs. per sq. in 55,000 

Elongation in 2 ins., per cent 20 

Small amounts of chromium are sometimes added to increase the 
elastic limit, so that in rolled sections the elastic limit will often be 
as high as 85,000. It is stated that the resistance to shock is not 
apparently lowered by the addition of chromium up to 1 per cent., 
but with chromium above 0.5 per cent, the elongation is rapidly 
decreased, and with chromium above 1 per cent, the elongation falls 
below 20 per cent, in 2 inches. 

The heat treatment of high-manganese steels presents a most 
important phase in connection with the successful application of 
these steels. Incorrect treatment is responsible for many of the 
failures which have been registered against manganese steels, and 
usually has been caused either by a mistaken idea of the particular 
structure best suited to the specific work in hand, or by a lack of 
sufficient knowledge of the mechanics of the austenite transforma- 
tion. The use of the microscope, and a judicious consideration 
and application of the results obtained, are probably the best means 
of solving a given problem in connection with heat-treatment 
adjustments. 

This thermal treatment for the majority of these steels involves 
two distinct, though correlated factors: (1) The change of grain 
size, and (2) the relationship of austenite and carbide, with or with- 
out the presence of martensite. As the principal manganese steels 
now used in commercial practice do not naturally contain mar- 
tensite, nor is it generally wanted, its consideration may be omitted. 

The necessity for the first requirement should be obvious: com- 
mercial high-manganese steel as cast is fundamentally austenitic; 
the crystals are often excessively large, and in many instances form 
a weak, columnar structure. It is evident that such a steel, for 
many purposes, will be entirely unsatisfactory. 



394 



STEEL AND ITS HEAT TREATMENT 



On the other hand, many high-manganese steels as forged, are 
characterized by an exceedingly fine, almost chalky structure, and 
yet may be very brittle. If a bar of manganese steel should be 
heated to 1800° F., and one half be allowed to cool slowly and the 
other half quenched in water, both ends will have a comparatively 
fine structure or grain size, yet the slow-cooled end will have but 
2 to 4 per cent, elongation as against 50 to 60 per cent, elongation 
in the quenched end. 

Although annealing will effect a change in the grain size, to 
a greater or lesser extent, it will also have a far-reaching, vastly 
more important, and injurious result — and we intentionally omit any 
reference to the tendency which certain compositions might have to 
become martensitic on very slow cooling. This effect of annealing 
is due to the formation, on very slow cooling, of the maximum 
amount of carbide — an extremely hard and brittle manganitic 
cementite rejected by the austenite, and which forms as a weak 
membrane around the austenite grains, as spines and needles, or 
in other characteristic manner. This is shown in the photomicro- 
graph in Fig. 259, taken from a rolled commercial manganese steel 




Fig. 259. — Commercial Manganese Steel Annealed at 1750° F. 
X100. (Bullens.) 

and then annealed at 1750° F. The effect of annealing upon the 
physical properties is shown by the following table of results, 
obtained from tests upon annealed manganese steels: 



MANGANESE, SILICON AND OTHER ALLOY STEELS 395 





High-Manganese Steels, Annealed. 




Carbon, 
Per Cent. 


Manganese, 
Per Cent. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elas tic 

Limit, 

Lbs. per Sq. In. 


Elongation 
in 2 Ins., 
Per Cent. 


Reduction 
of Area. 
Per Cent. 


0.95 
1.00 
1.07 


10.07 
11.21 
13.38 


94,600 

99,950 

103,670 


67,500 
77,660 
62,350 


1 
1 
3 


1.4 

0.75 

3.4 



Annealing these high-carbon manganese steels is obviously 
illogical, and it is the carbide which is the main source of the diffi- 
culty. And yet a large proportion of the peculiar and distinctive 
wearing qualities of these steels is probably due to the manganitic 
carbide, although in a form other than that just described. 

On heating to a high temperature the carbide membrane produced 
by slow cooling, or the carbide segregations, are gradually taken 
into solution by the austenite; and by rapid cooling from that same 
temperature the carbide is more or less prevented from reprecip- 
itating, and especially from taking on that enweakening structure 
(i.e., as a membrane or segregations) previously mentioned. Since 
the carbide originally formed in casting is very sluggish in its response 
to heat in being absorbed by the austenite (as is also a character- 
istic more or less marked in all hyper-eutectoid steels), and as the 
equalization of the steel as a whole also takes place slowly, a high 
temperature is necessary. Further, the temperature must also be 
high, and the cooling be effected very rapidly — such as water quench- 
ing — to retain the carbide in solution. Such a treatment will give 
the most ductile steel. The temperature required is generally not 
less than 1830° F. Thus a water quenching from 1830° F. of the 
annealed steels previously given will show a tensile strength of about 
135,000 to 145,000 lbs. per square inch, with an elongation of 50 
to 60 per cent. 

On the other hand, by varying the factors of temperature, 
duration of heating, and rate of cooling, it is possible to obtain phys- 
ical properties covering a wide range. The static strength and 
ductility are largely governed by the amount of the original free car- 
bide which is taken into solution and there retained by water quench- 
ing. Thus the properties of the steel, looking at the results of heat 
treatment from this point of view, may be varied from that charac- 
teristic of the steel as cast, rolled, or forged, to that indicative of a 
full " water toughening." 

High-manganese steel has no critical or transformation points or 



396 STEEL AND ITS HEAT TREATMENT 

ranges. Thus while in the ordinary steels the heat treatment is 
more or less guided by such temperatures, in high-manganese steels 
the only criterion of proper temperatures is the relation of the car- 
bide to the physical properties : the absorption, with or without the 
precipitation, of such carbide, is the underlying basis for heat-treat- 
ment adjustment. 

SILICON AND SILICO-MANGANESE STEELS 

The use of silicon in commercial steels is practically confined 
to two classes: (1) a medium carbon and about 1.50 per cent, silicon, 
for use in tempered gears and springs — known as silico-manganese 
steel; and (2) a nearly carbonless steel with up to 3.50 per cent, 
silicon, for use in electrical apparatus. 

The manufacture of silico-manganese steels in the open hearth 
must be carefully watched on account of their great tendency to 
piping and segregation, and a large discard must be made in the 
cropping of the ingots- to insure good steel. Silico-manganese steels 
have considerable popularity among the' foreign automobile manu- 
facturers; their use in this country, however, is usually limited to 
the lower-priced cars. Although the lower cost favors their use, 
their great sensitiveness to heat treatment and feeble resistance to 
shock limits their field of usefulness. But when handled with great 
care the silico-manganese (and also the silico-chrome) steels will 
give good results in works well equipped for obtaining accurate 
results in their heat-treatment operations. The temperature limits 
for quenching are narrower than for most alloy steels, and the steel 
responds altogether too quickly to variations in heating and cooling. 
Although these steels will give high static test results upon suitable 
treatment, the sensitiveness which is inherent to this type of steel 
usually proves the governing factor. 

A typical American analysis for silico-manganese steel for gears 
and springs is as follows: 

Carbon, per cent 0.43 to 0. 53 

Manganese, per cent . 5Q to . 70 

Silicon, per cent 1 . 25 to 1 . 50 

Upon suitable treatment, usually a quenching in oil from about 1550° 
to 1600° F., followed by tempering to suit the requirements, the fol- 
lowing results are representative : 

Tensile strength, lbs. per sq. in 195,000 to 230,000 

Elastic limit, lbs. per sq. in 175,000 to 220,000 

Elongation, per cent, in 2 ins 12 to 8 



MANGANESE, SILICON AND OTHER ALLOY STEELS 397 



The tables below (taken from a work by Revillon) give some 
characteristic French and German gear steels of the silicon-man- 
ganese type, together with their physical properties. Steel No. 6 
(a straight carbon steel) is given for comparison, and was designed 
to do away with the reheating or tempering necessary with the 
high-silicon steels; it is reported that tests on the untempered gears 
gave very satisfactory results. 

As a side-light on the effect of certain treatments of pieces of 
large section of silico-manganese steel, the following may be of 
interest. Experiments were made with a characteristic steel, 5 ins. 
in diameter, and analyzing 0.44 per cent, carbon, 0.60 per cent, 
manganese, and 1.50 per cent, silicon. The purpose was to deter- 
mine the possibility of using this steel in place of a 0.60 per cent, 
chromium steel with approximately the same manganese and carbon 
content. The principal requirement to be met was to obtain a glass- 
hard surface such as could be obtained with the chromium steel. It 

Experiments with Silico-manganese Gear Steels 





Chemical Analysis. 


Annealed at 1650° F. 


No. 


Car- 
bon. 


Man- 
ganese 


Silicon 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation 

% 


Reduce 

tion 

of 

Area. 

% 


Brinell 
Hard- 
ness. 


Shock 
Test. 


1 
2 
3 
4 
5 


0.40 
0.57 
0.50 
0.70 
0.39 


0.57 
0.61 
0.64 
0.78 
0.52 


1.89 
1.22 
1.64 
1.87 
1.98 


105,820 
127,860 
123,450 
142,510 
109,230 


68,410 
74,100 
82,490 
80,790 
78,510 


19 

15 

12.5 

15.5 

20 


40 
29 
21 
35 
43 


207 
225 
215 
241 
203 


36.1 
21.7 
32.5 
23.3 
43.4 


6 


0.45 


0.42 


0.43 


87,760 


48,780 


19.5 


51 


197 


43.4 




Quenched oil from 1520° F., drawn at 930° F.* 


No. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic Limit, 
Lbs. per Sq. In. 


Elongation. 


Reduction 
of Area. 


Brinell 
Hardness. 


Shock 
Test. 


1 

2 
3 

4 
5 


178,380 
219,460 
156,450 
210,500 
135,120 


159,300 
204,500 
136,830 
197,700 
107,810 


4.5 
5.5 
4 

2.5 
11 


12 
15 
19 
17 
43 


315 
467 
307 
435 
274 


39.8 
25.3 
75.9 

47 
47 


6 


278,770 


271,660 


3.1 


14 


422 


39.8 



* Nos. 5 and 6 were quenched from 1560° F. ; No. 6 was drawn at 400° F. 
The critical ranges of No. 5 were: Ac, 1560°; Ar, 1410°. For No. 6, Ac, 1420°; 
Ar, 1290°. 



398 



STEEL AND ITS HEAT TREATMENT 



was found that the hardness requirement could not be met with the 
silico-manganese steel, and more important, that the steel would 
often split or crack when quenched in either the cold water or brine 
bath used for the chromium steel. 

The most important use for straight silicon steels is that for 
electromagnets and for other electrical purposes demanding a high 
magnetic permeability or electrical resistance. Hadfield's silicon 
steel, containing approximately 2.75 per cent, silicon, and with car- 
bon, manganese and the other impurities as low as possible, is repre- 
sentative of this class. His treatment for this steel consists of first 
heating it to about 1950° F. and cooling quickly, and then heating 
to 1380° F. and cooling very slowly, and which is sometimes fol- 
lowed by a reheating to 1475° F. and cooling very slowly. 

Another silicon steel, used in place of dynamo sheet iron, specifies 
similar carbon, manganese, etc., but with a silicon content of about 
3.25 per cent. The thermal treatment recommended for this steel 
is a thorough heating at about 1430° to 1475° F., followed by very 
slow cooling. 

TUNGSTEN STEELS 

The pearlitic low-tungsten steels when quenched from the proper 
temperature do not appear to be any more modified by this quench- 
ing than are the corresponding straight carbon steels; the effect of 
tungsten in such steels is, however, to increase the tensile strength, 
with the degree of brittleness remaining about the same. For this 
reason tungsten is sometimes used in place of silicon — which has a 
feeble resistance to shock — for springs. The following table gives 
the analysis and physical properties of a characteristic low-tungsten 
spring steel: 



Carbon 0.45% 

Manganese . 22% 



Silicon 0.30% 

Tungsten 0.60% 



Tensile strength, lbs. per sq. in 
Elastic limit, lbs. per sq. in.. . . 
Elongation, per cent 



Annealed. 



113,500-121,000 

85,000 

14 



Quenched in Oil from 

1560°, Drawn at 

930° F. 



185,000 
128,000 

7 



The use of tungsten for ordinary structural purposes is mainly 
limited by the fact that such steels have to be made by the crucible 
process. 



MANGANESE, SILICON AND OTHER ALLOY STEELS 399 

The other, and most important uses for tungsten, are those for 
permanent magnets (the steel usually being used in the hardened 
condition), and for various varieties of tool steels in both high- 
speed and water- or oil-hardening types. High speed steels are dis- 
cussed in Chapter XVIII. 

MOLYBDENUM STEEL 

On account of its high cost the use of molybdenum has been 
largely confined to high-speed and similar steel — and even there it 
has usually been superseded by tungsten. In the lower percentages, 
molybdenum may be present in steel as a fairly easily decomposable 
iron-molybdenum compound; with larger amounts of both molyb- 
denum and carbon it is generally believed that the molybdenum 
forms a double carbide in a similar manner to chromium. 

In the pearlitic molybdenum steels the influence of molybdenum 
is much like that of chromium, in that it increases the tendency to 
greater hardness with proper quenching or with cold work, and like- 
wise to increased brittleness upon prolonged heating at high temper- 
atures. On the other hand, the molybdenum steels have a markedly 
higher ductility and toughness, besides an increased dynamic 
strength. The best results (disregarding its use for tools) have 
been obtained with the use of 1 to 2 per cent, molybdenum, in 
combination with the proper proportion of carbon — the carbon having 
a marked influence upon the physical properties of molybdenum 
steels. Considerable experimentation has been carried out with 
pearlitic molybdenum steels for rifle barrels and large guns; it has 
also been used in high-duty machine parts such as propeller-shaft 
forgings. It is reported that excellent results have been obtained 
in case-hardening steels with about 1 per cent, molybdenum. 

The influence of molybdenum depends largely upon the heat 
treatment, as is shown in the following series of tests by Swinden r 
with steels containing 1, 2, 4, and 8 per cent, molybdenum. 

1 Thos. Swinden, Carnegie Scholarship Mem., Vol. Ill, 1911. 



400 



STEEL AND ITS HEAT TREATMENT 

1.00 Per Cent. Molybdenum Steel 





Chemical. 


As Rolled. 


No. 


C. 


Mo. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic 

Limit, 

Lbs. per Sq. In. 


Elongation, 
Per Cent. 
in 2 Ins. 


Reduction 
of Area 
Per Cent. 


1 

2 
3 
4 


0.195 
0.445 
0.87 
1.215 


1.03 
1.05 
1.02 
1.10 


67,040 
108,800 
160,000 
117,340 


44,800 

78,800 

104,000 


33.31 

19.5 

14.5 

1.0 


64.32 

49.23 

34.36 

2.02 









Annealed 



No. 


Tensile 
Strength, 
Lbs. per 

Sq. in. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation 

Per Cent. 

in 2 Ins. 


Reduc- 
tion of 
Area, 
Per Cent. 


Bend 
Test, 
Deg. 


Alter- 
nat- 
ing 
Str'gth 


Brinell 
Hard- 
ness. 


Sclero- 
scope 
Hard- 
ness. 


1 

2 
3 
4 


52,300 

71,420 

108,100 

85,300 


27,800 
38,720 
52,000 
52,100 


35.5 
25.0 
17.22 
5.55 


65.75 

39.2 

22.25 

7.5 


180 

180 

67 

25 


. 336 

210 

103 

14 


99 
131 

228 
207 


11 
13 
23 
22 



Heat Treated (Hardened in oil, reheated to 1025° F.). 



1 


90,100 


47,150 


27.46 


68.4 


180 


301 


241 


27 


2 


210,560 


168,600 


14.08 


49.2 


180 


137 


387 


37 


3 


240,490 


193,700 


9.15 


25.2 


16 


92 


418 


44 


4 


279,000 


203,900 


4.92 


12.0 


34 


71 


512 


45 



MANGANESE, SILICON AND OTHER ALLOY STEELS 401 
2.00 Per Cent. Molybdenum Steel 





Chemical. 


As Rolled. 


No. 


C. 


Mo. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic 

Limit, 

Lbs perSq. In. 


Elongation, 

Per Cent. 

in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


5 


0.246 
0.442 
0.883 
1.21 


2.176 
2.181 
2.186 
2.109 


117,820 
150,980 
198,910 
216,830 




21.05 
16.7 
12.1 
7.04 


57 


6 




46 41 


7 
8 


124,250 
169,340 


32.07 
9.6 



Annealed 



No. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation 
Per Cent, 
in 2 Ins. 


Reduc- 
tion of 
of Area, 
Per Cent. 


Bend 
Test, 
Deg. 


Alter- 
nat- 
ing 

Str'gth 


Brinell 
Hard- 
ness. 


Sclero- 
scope 
Hard- 
ness. 


5 
6 

7 
8 


65,070 

82,300 

107,070 

95,200 


31,580 
43,400 
54,770 
61,710 


33.3 

27.7 

18.8 

9.4 


62.5 
44.3 
27.5 
13.5 


180 

180 

100 

43 


370 
259 
126 

27 


116 
143 
207 
196 


15 

18 
22 
22 



Heat Treated (Hardened in oil, reheated to 1025° F.). 



5 


171,140 


115,400 


15.49 


54.4 


180 


172 


387 


35 


6 


211,460 


149,100 


14.08 


47.2 


180 


103 


444 


39 


7 


260,800 


178,800 


5.63 


12.0 


26 


80 


512 


47 


8 


270,940 








16 


39 


512 


48 



4.00 Per Cent. Molybdenum Steel 





Chemical. 


As Rolled. 


No. 


C. 


Mo. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic 

Limit, 

Lbs. per Sq. In. 


Elongation, 
Per Cent, 
in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


9 
10 
11 


0.19 
0.487 
0.865 
1.06 


4.11 
4.01 
4.00 
4.02 


119,120 
188,160 
230,270 
239,230 


75,370 
120,060 


21.70 

13.5 

8.0 

10.56 


52.71 
33.81 
17.27 


12 


179,900 


18.40 



402 



STEEL AND ITS HEAT TREATMENT 

Annealed 



No. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation, 
Per Cent, 
in 2 Ins. 


Reduc- 
tion of 
Area, 
Per Cent. 


Bend 
Test, 
Deg. 


Alter- 
nat- 
ing 

Str'gth 


Brinell 
Hard- 
ness. 


Sclero- 
scope 
Hard- 
ness. 


9 
10 
11 
12 


63,390 
77,060 
94,300 
92,960 


31,470 
42,220 
45,920 
42,560 


42.7 
28.3 
20.5 
15.5 


72.5 
52.0 
34.0 
20.5 


180 

180 

180 

94 


366 

247 

146 

66 


116 
143 
179 
196 


17 
18 
20 
23 



Heat Treated {Hardened in oil, reheated to 1025° F.) 



9 


88,180 


66,500 


30.20 


64.0 


180 


329 


286 


28 


10 


188,900 


139,780 


11.26 


41.6 


123 


109 


444 


43 


11 


258,050 


203,940 


4.22 


4.8 


56 


52 


512 


44 


12 


282,240 


267,000 


7.04 


23.2 


4 


45 


532 


48 



8.00 Per Cent. Molybdenum Steel 





Chemical. 


As Rolled. 


No. 


C. 


Mo. 


Tensile 

Strength, 

Lbs. per Sq. In. 


Elastic 

Limit, 

Lbs. per Sq. In. 


Elongation, 
Per Cent, 
in 2 Ins. 


Reduction 
of Area, 
Per Cent. 


13 


0.135 
0.361 
0.445 
0.775 
1.125 


8.01 
8.17 
8.11 
7.85 
7.92 


92,290 
148,290 
215,040 
193,890 
245,500 




25.7 
19.4 
19.71 
9.85 
8.45 


52.22 


14 




45.9 


15 
16 
17 


154,110 
149,090 
189,950 


34.0 
18.4 
16.4 



Annealed 



No. 


Tensile 
Strength, 
Lbs. per 

Sq. In. 


Elastic 

Limit, 

Lbs. per 

Sq. In. 


Elon- 
gation, 
Per Cent, 
in 2 Ins. 


Reduc- 
tion of 
Area, 
Per Cent. 


Bend 
Test, 
Deg. 


Alter- 
nat- 
ing 

Str'gth 


Brinell 
Hard- 
ness. 


Sclero- 
scope 
Hard- 
ness. 


13 


79,070 


41,660 


31.1 


58.75 


180 


283 


143 


16 


14 


77,060 


34,720 


36.6 


68.23 


180 


273 


143 


18 


15 


83,220 


38,640 


32.2 


57.5 . 


180 


215 


156 


18 


16 


87,580 


45,140 


22.2 


35.5 


171 


108 


170 


20 


17 


92,290 


48,830 


16.1 


24.0 


85 


66 


187 


22 



Heat Treated {Hardened in oil, reheated to 1025° F.) 



13 


82,080 


53,760 


30.9 


65.6 


180 


239 


163 


15 


14 


105,350 


75,450 


25.3 


54.4 


180 


226 


351 


30 


15 


127,400 


77,800 


21.1 


49.2 


180 


122 


444 


39 


16 


247,300 


216,830 


7.74 


23.2 


34 


33 


512 


42 


17 










4 


24 


512 


46 



CHAPTER XVIII 

HIGH-SPEED STEELS 

High-Speed Steels. — The use of high-speed steel has revolu- 
tionized machine shop practice in that tools made from such steel 
possess the important property of " red-hardness," being able to 
maintain a sharp cutting edge while heated to temperatures which 
would ruin the best of carbon-steel tools. This ability to retain 
hardness at a red heat has resulted in an enormous increase in cut, 
feed and turning speed. Such properties are obtained by a correla- 
tion of suitable chemical composition and heat treatment. 

Chemical Composition. — In addition to the standard ingredients 
of carbon tool steel, such as carbon, manganese, silicon, phosphorus 
and sulphur, the principal alloying elements in high-speed steel 
are tungsten, chromium and vanadium, and in some instances 
cobalt or molybdenum. The following table of chemical composi- 
tions relates to current high-speed steel analyses such as are gen- 
erally used for heavy duty at high speeds, and for finishing when a 
perfect surface is not required. 

Per cent. 

Carbon 0.55 to 0.75 

Manganese under . 35 

Phosphorus under . 015 

Sulphur under . 02 

Silicon 0.10 to 0.30 

Tungsten 16 to 20 

Chromium 2 to 5 

Optional 

Vanadium 0.25 to 1.00 

Cobalt 2.5 to 5 

403 



404 



STEEL AND ITS HEAT TREATMENT] 



An average of analyses of over twenty different makes of American 
high-speed steels gave: 

Per cent. 

Carbon 0.67 

Manganese . 27 

Silicon 0.23 

Tungsten 16.5 

Chromium 4.3 

Vanadium . 82 



It appears to be the tendency of foreign manufacturers to use 
greater amounts of the alloys, especially tungsten, as will be seen 
from the following table of compositions : 



Bedel & Co., Paris. . 
Austrian Phoenix. . . 

English Novo 

Austrian high-speed. 

Universal 

Kysar 

Armstrong 

German 



c 


Mn 


Si 


w 


0.90 


0.27 


0.20 


22.80 


0.67 


0.14 


0.15 


20.70 


0.76 


0.42 


0.33 


18.85 


0.93 


0.23 


0.24 


24.50 


0.60 


0.14 


0.42 


23.50 


0.87 


0.27 


0.11 


18.77 


0.78 


0.49 


0.44 


12.44 


0.60 


0.24 


0.23 


30.20 



Cr 

8.10 
3.70 
2.95 
7.19 
6.50 
1.22 
3.40 
3.70 



One of the recent German steels, containing about all the alloying 
elements used, analyzed as follows: 



Per cent. 

Carbon 0.72 

Manganese . 08 

Phosphorus 0.010 

Sulphur 0.010 

Silicon 0.35 



Per cent. 

Tungsten 16.18 

Chromium 4.3 

Vanadium 0.87 

Cobalt 4.00 

Molybdenum... 0.92 



Effect of Tungsten. — The effect of tungsten in high-speed steels 
may be attributed, among other things, to the following factors: 
(1) as a strong obstructing agent to the transition of austenite to 
pearlite; (2) as a fixing agent for martensite, thus conferring the 
property of "red-hardness"; (3) additional hardness imparted by 
the double carbide of tungsten and iron. 

When a steel containing 0.65 per cent, carbon and over 6 or 7 
per cent, tungsten is given a moderately rapid cooling, such as air 



HIGH-SPEED STEELS; 



405 



quenching, from a high temperature the change to pearlite is pre- 
vented. Only by extremely slow cooling can the pearlitic or, as 
is more usual, the sorbitic structure be obtained in high-tungsten 
steels. Depending upon the temperature to which the steel is 
heated and upon the rate of cooling from that temperature the steel 
may be austenitic, martensitic or troostitic in character. That 
this temperature must be high in order to produce self-hardening is 
shown by the fact that a very rapid air quenching from temperatures 
up to 1925° F. had practically no hardening effect on a steel contain- 
ing 19.28 per cent, tungsten with 0.63 per cent, carbon. But by 
increasing the temperature to 2460° F. and air quenching a Brinell 



600 

a 

£500 

n) 

K 
moo 

300 



































500 1000 1500 

Temperature of Re-heating after Hardening 



Fig. 260. — Effect of Drawing on Hardness of High Tungsten Steel. 



hardness number of about 500 was obtained. Even under these 
latter conditions, however, the steel was much softer than a similar 
specimen containing 6 per cent, chromium which had been slowly 
cooled in air. 

The most important action of tungsten in the absence of chro- 
mium is to raise the temperature at which tempering or annealing 
begins — that is, the temperature at which martensite will begin 
to change into troostite and sorbite. Thus in a straight carbon 
tool steel a heating (after hardening) to 400° or 500° F. will cause 
tempering to take place, and a heating to 1100° F. will more or less 
soft-anneal such a steel and make it unfit for use as a cutting tool. 
The presence of a considerable percentage of tungsten in the steel, 



406 STEEL AND ITS HEAT TREATMENT 

however, not only permits a retention of the hardness incurred by 
rapid cooling, but also, under proper conditions, considerably 
increases the hardness when the steel is reheated to a temperature 
as high as 1100° F. This is illustrated by the chart in Fig. 260 
in which the Brinell hardness numbers have been plotted from 
experimental data of Edwards and Kikkawa 1 upon a steel contain- 
ing 0.63 per cent, carbon, 19.28 per cent, tungsten and no chromium, 
air quenched from 2460° F. and reheated to the temperatures 
indicated. It will be seen that at no temperature up to 1150° F. 
does the Brinell hardness of the reheated steel fall below that of the 
hardened steel, and further, that with the treatment at about 1100° 
F. the hardness is greatly increased. This means that it is possible 
to use a speed of cutting 2 such that the point of the tungsten steel 
tool will reach a dull red color and still retain its hardness for at least 
a considerable time. Thus we have the property of " red-hardness " 
conferred by tungsten. 

Effect of Chromium. — The presence of chromium greatly increases 
the secondary hardness which is brought about by the low heat 
treatment, as is shown by the table 3 on page 407. In the case 
of these chromium-tungsten steels it will be noted that the first 
effect of the reheating is to make them softer, but when they are 
drawn at higher temperatures they again become harder than in the 
initial air-quenched state. From these and other experiments it 
was also concluded that chromium is not only the cause of the great 
hardness of high-speed steels, but also produces a marked lowering 
of the temperatures at which hardening can be effected. This 
latter is brought out by the position of the minimum Brinell hard- 

1 Edwards and Kikkawa, Inst. Journ., 1915. 

2 Edwards states that although the hardness was determined at ordinary- 
temperatures, he thought that there was no doubt but that the figures represent 
the " red-hardness"; from which we might infer that he refers also to the high- 
speed cutting efficiency. On the other hand, Arnold (J. O. Arnold, Chem. 
News, 113, pp. 218-219, 1916), from data obtained with several tungsten- 
chromium and tungsten-chromium-vanadium steels, states that there is no 
relationship between Brinell hardness and lathe efficiency; the latter depends 
not so much upon actual hardness as upon thermal stability of the simple or 
compound hardenites in the hardened steel; that the simple hardenite of a 
carbon steel has a thermal stability of which the limit is certainly less than 
575° F., but the compound hardenite of a high-speed steel may be rendered 
stable up to 1292 (?)° F. In other words, there is no doubt of the value of 
" red hardness " conferred by tungsten, whether or not it is definitely related to 
Brinell hardness. 

3 Edwards and Kikkawa, Inst. Journ., 1915. 



HIGH-SPEED STEELS 



407 



ness obtained by reheating these steels and the rapidity with which 
hardness is again obtained by the use of higher drawing heats. 
Thus the minimum hardness of the steel without chromium was 
obtained at 1918° F., and that of the 6.24 per cent, chromium 
steel at 1443° F.; reheating the latter steel to 1918° F. nearly 
doubled the hardness as obtained at 1443° F. 



THE 


BRINELL HARDNESS 


OF 


HEAT-TREATED CHROMIUM-TUNGSTEN 


STEELS 


1 
Steel air quenched from 2460° F. and reheated as shown. 


Chemical 
Composition. 


Hard. 


405 


576 


747 


833 


921 


1008 


1092 


1137 


1177 


1267 


1355 


1443 


1544 


1634 1724 


1859 


1918 


C 


Cr 


W 


499 


512 


526 


512 


522 


503 


512 


635 


564 


611 


535 


435 


364 


333 


321 


325 


321 


313 


0.63 




19.28 


700 


71 a 


6SS 


700 


700 


652 


688 


700 


708 


676 


573 


457 


402 


364 


351 


368 


382 


398 


0.63 


1.12 


19.40 


719 


727 


700 


70S 


700 


700 


700 


744 


727 


688 


622 


474 


406 


402 


330 


560 


643 


606 


0.68 


3.01 


19.37 


727 


713 


tiSS 


700 


700 


70S 


719 


752 


700 


652 


611 


471 


382 


364 


543 


582 


611 


611 


0.64 


4.91 


19.33 


708 


727 


700 


700 


6SS 


700 


700 


752 


700 


643 


578 


448 


382 


373 


530 


643 


648 


659 


0.67 


5.99 


18.86 


694 


688 


659 


665 


652 


637 


643 


756 


727 


694 


592 


426 


385 


409 


535 


632 


643 


643 


0.64 


6.24 


17.69 



Maximum and minimum hardness shown by heavy-face type. 

Critical Ranges of High-speed Steel. — Sauveur and Yatsevitch, 1 
from critical range experiments upon high-speed steel of the com- 
position 

Per cent. 

Carbon 0.62 

Manganese . 62 

Silicon 0.27 

Tungsten 18.20 

Chromium 3 . 00 

Vanadium 0.95 

have found that this steel exhibits on heating two critical points at 
about 1425° and 1570° F. respectively, and which were found to be 
remarkably constant. The curves obtained on cooling, however, 
showed surprising results. Upon slow cooling from a temperature 
not exceeding 1750° F. the curves indicated but one critical point 
at about 1345° to 1365° F. But by increasing the maximum tem- 
perature to which the steel is heated and the rate of cooling 
this critical point of the cooling curve is nearly, if not entirely, 
suppressed, while a lower critical point appears and grows enormously 
in intensity. The experimenters then conclude that " the inference 
appears warranted that the treatment imparting high-speed hard- 

1 Sauveur, " The Metallography and Heat Treatment of Iron and Steel," 
2d ed., p. 357. 



408 STEEL AND ITS HEAT TREATMENT 

ness suppresses, at least for the major part, a certain transformation 
occurring normally on slow cooling from moderately high tem- 
peratures at about 1345° F. while inducing with increasing magnitude 
a new transformation generally between 660° and 750° F. The 
conclusion that the lower point is merely the upper point depressed 
by high heating and quick cooling would be untenable, since the 
cooling curves show conclusively that the upper point is not lowered 
but merely decreases in intensity. As to the nature of the trans- 
formation apparently suppressed and the transformation induced 
by the high-speed treatment, any positive statement would be 
premature. We naturally connect the upper point with the separa- 
tion of carbide on slow cooling, but the cause of the lower point is 
still a matter for further investigation." 

Taylor-White Method. — The original Taylor-White method 
for treating high-speed steel consisted in heating the tool to the 
highest temperaure it will bear, generally near the melting-point 
if it can be ground afterwards. The tool was then plunged into a 
lead bath maintained at a constant temperature of 1150° F., held 
there until the steel had reached that temperature throughout its 
mass, and then permitted to cool from that temperature. The 
second step or low-heat treatment consisted of heating the tool to 
a temperature preferably about 1150° F. for a period of about five 
minutes and cooling to the temperature of the air either rapidly or 
slowly. 

Taylor's own remarks 1 on the subject of various methods of 
hardening high-speed steel tools are particularly interesting, and 
although written a decade ago are most pertinent to-day: 

" For some years past it has been rather amusing to us to hear 
the special directions given by the various manufacturers of steel 
suitable in chemical composition for making the high-speed tools. 
Very frequently a tool-steel maker implies, or directly states, that 
the chemical composition of his particular high-speed tool steel 
requires ' special treatment.' The fact is, however, that our 
recent experiments demonstrate beyond question the fact that no 
other method which has come to our attention produces a tool 
superior in red hardness (i.e., high-speed cutting ability), or equal 
in uniformity to the method described. This applies to all makes 
of high-speed tool steels which are capable of making first-class tools, 
whatever their chemical composition. 

1 F. W. Taylor, " On the Art of Cutting Metals," Trans. Am. Soc. Mech. 
Eng., Vol. XXVIII, 1906, pp. 200-201. 



HIGH-SPEED STEELS 409' 

" It is the writer's belief that during our long series of experiments 
at the Bethlehem Steel Company, in our search for uniform tools 
and for the method of imparting the highest degree of red hardness 
to tools, we tried substantially every method which has since come 
to our attention. 

" For instance, in giving the tools the high heat we heated them 
in a blacksmith's soft-coal fire, in muffles over a blacksmith's fire, 
and in gas-heated muffles. We also constructed various furnaces 
for this purpose. We heated tools by means of an electric current, 
with noses under water, and out of water, and by immersion in 
molten cast iron. Moreover, by every one of these methods we were 
able to produce a first-class tool, provided only the tool was heated 
close to the melting-point. 

" In cooling from the high heat we experimented with a large 
variety of methods. After being heated close to the melting-point, 
tools were immediately buried in lime, in powdered charcoal, and in 
a mixture of lime and powdered charcoal; thus they were cooled 
extremely slowly, hours being required for them to get below a 
red heat. And we wish clearly to state the fact that tools cooled 
even as slowly as this, while they were in many cases quite soft and 
could be filed readily, nevertheless maintained the property of 
' red-hardness ' in as high a degree as the very best tools, and were 
capable of cutting the medium and softer steels at as high-cutting 
speeds as the best tools which were cooled more rapidly and which 
were much harder in the ordinary sense. 

" Tools were also cooled from the high heat in a muffle or slow- 
cooling furnace with a similar result. On the other hand, we made 
excellent high-speed tools by plunging them directly into cold water 
from the high heat, and allowing them to become as cold as the water 
before removing them. Between these two extremes of slow and fast 
cooling — cooling in lime, charcoal, or a muffle, on the one hand, and 
in cold water on the other — other cooling experiments covering a 
wide range were conducted. We tried cooling them partly in water 
and slowly for the rest of the time ; partly in oil, and then slowly for 
the rest of the time; partly by a heavy blast of air from an ordinary 
blower and the rest of the time slowly; partly under a blast of com- 
pressed air and then slowly. We also reversed these operations by 
cooling first slowly and then fast, as described, we also cooled them 
entirely in an air blast and entirely in oil, and then partly first 
in oil, afterward in water, and then first in water and afterward in 
oil. 



410 STEEL AND ITS HEAT TREATMENT 

" By every one of these methods we were able to make 
a good high-speed tool; i.e., a tool having a large degree of 
red-hardness, and capable of cutting at very high cutting speeds. 
But by none of these processes were we able to obtain tools as 
uniform and regular as those produced by our lead bath and 
air cooling." 

Microstructure. — The general aim in hardening high-speed 
steel should be to obtain the so-called austenitic or polygonal struc- 
ture, free from particles of carbide. The free carbide is character- 
istic of annealed high-speed steel, and is generally more or less present 
in the hammered steel. In order to accomplish the solution of such 
carbide the temperature must be high and yet not high enough to 
burn the steel. The effect of trying to harden from too low a tem- 
perature is to produce a structure with particles of carbide imbedded 
in a martensitic ground-mass instead of the polygonal structure. 

American Practice. — The prevalent American practice of 
hardening high-speed' steel first is to warm the tool, slowly and 
carefully raising the temperature to about 1500° F., and then more 
rapidly to the high temperature, followed by a quenching in oil or 
in the air blast. The secondary or low-heat treatment is not 
generally followed except in some cases for special milling cut- 
ters and similar tools. A hardening temperature of 2275° to 
2325° F. is recommended for the standard American high-speed 
steels. 

Pieces to be hardened all over or pieces with fine cutting edges are 
advantageously brought to the quenching temperature by immer- 
sion in white hot lead after the necessary preliminary heating 
to color, or in a bath of barium chloride maintained at the proper 
temperature, and then quenched in oil free from water until the oil 
ceases to flash on the surface of the piece. A little potassium 
ferrocyanide added to the bath will tend to prevent decarburization 
of the cutting edges while heating. 

High-speed steels may be annealed by a thorough saturation at 
about 1500° F. (for some steels as high as 1600° F.) followed by 
a very slow cooling. Annealing in pipes, tubes or boxes, with a 
packing of charcoal, ashes or a combination of the two, is recom- 
mended. 



CHAPTER XIX 

TOOL STEEL AND TOOLS 

The problem of selecting a proper grade of steel in relation to 
the work required is one hitherto met by the steel manufacturer 
alone. Until recently he has recommended this or that steel for a 
given requirement, depending more or less upon his general knowl- 
edge of the purpose for which the tool is to be used, and upon the 
experience of his customers in the past. But with the entrance of 
the technical man into manufacturing concerns and the great im- 
provements resulting therefrom, a fuller knowledge of various steels, 
their composition, applicability and efficiency has been demanded. 
This has resulted in a wider dissemination of information regarding 
the physical, chemical and mechanical properties of steels manu- 
factured by various steel companies, and a corresponding education 
of both maker and buyer. 

Grade.- — For the aid and information of their customers, the steel 
maker usually groups his tool-steel products into various " grades " 
and " tempers." The former term refers to the " quality " of the 
steel, according to the class of raw material which has been used, 
together with the skill and care taken in producing the finished 
material. The highest grades should be used for tools operating 
under severe working conditions, demanding great endurance and 
resistance to torsional or other strains, or upon which a large labor 
cost has been placed. These conditions, such as are found in expen- 
sive dies, milling cutters, taps, etc., would require a high-grade 
steel. For such purposes as mill-picks, cheap tools, etc., it would 
be folly to use any but a lower-quality steel. Wear, the cost of 
redressing, regrinding and heat treatment are other factors which 
must be considered in the selection of the proper and most economical 
steel which will give the greatest efficiency in all senses of the word. 
With this in mind, the following brief synopsis is given : 

1. Finest tools and dies: expense for material the smallest item 
entering into the cost and upkeep of the finished tool; 

2. Finishing tools for lathe and planer work; special taps, 

411 



'412 STEEL AND ITS HEAT TREATMENT 

reamers, milling cutters and other similar tools requiring a high- 
grade steel; wood-working and corrugating tools; 

3. General tool purposes; 

4. Ordinary purposes, such as chisels, smith and boiler shop 
work, etc. 

5. For rough or heavy work. 

Expressing this in a different way, we may say that the choice 
of a grade of tool steel depends upon three factors : 

1. The precision of the work required of the tool; 

2. The relative cost of the steel in comparison with the labor 
involved in the manufacture of the tool; 

3. The life of the finished tool and its relation to the cost of pro- 
duction. 

Temper. — Carbon tool steels are further denoted by the " tem- 
per." In tool-steel sales parlance this refers to the percentage of 
carbon in the steel and may be denoted by figures or letters. Such 
classifications generally refer to a 10-point carbon limit — thus No. 7 
temper may refer to 0.65 to 0.75 per cent, carbon, or it may be 
represented by whatever the individual company has arbitrarily 
selected. In this connection it should be noted that this " temper " 
does not refer to, and should not be confused with the word temper 
as indicating the operation of " letting down " the steel after 
hardening. 

General recommendations for the proper carbon content to use 
for various tools are given in the following table; these, however, 
must not be regarded as absolute, for much will depend upon the 
grade of steel and upon the exact use of the tool, 



APPROXIMATE CARBON CONTENT FOR ORDINARY TOOLS 

C ar k° n > Tools 

Per Cent. 

1.50 Tools requiring extreme hardness. For turning chilled- 
rolls and tempered gun-forgings. Roll corrugating. 
1.40 Hard lathe work generally. Chilled-roll' turning. Cor- 
rugating. 
Graver tools. 
Brass-working tools. 
1 . 30 General lathe, slotter and planer tools. 
Razors. 
Drawing dies. 



TOOL STEEL AND TOOLS 413 

p Car £ on ' Tools 

Per Cent. 

Mandrels, granite points, scale pivots, bush hammers, 
peen-hammers. 

Ball-races. 

Files. 

Trimming dies. Cutting dies. 
1.20 Twist drills. Small taps. 

Screw dies, threading dies. 

Edge tools generally. Cutlery. 

Cold stamping dies, leather-cutting dies, cloth dies, glove 
dies. 

Nail dies, jewelers' rolls and dies. 
1 . 10 Milling cutters and circular cutters of all descriptions. 

Wood-working tools, forming tools, saws, mill picks, axes. 

Small punches. 

Taps. 

Cup and cone steel. 

Small springs. Anvils. 
1.00 Reamers, drifts, broaches. 

Large milling cutters, saw swages. 

Springs. 

Mining drills, channeling drills. 

Large cutting and trimming dies. 
0.90 Hand chisels, punches. 

Drop dies for cold work, small shear knives. 

Chipping chisels. 

Cutting and blanking punches and dies. 
0.80 Large shear knives, chisels, hammers, sledges, track chisels. 

Cold sets, forging dies, hammer dies, boiler-maker's tools. 

Vise-jaws. Oil-well bits and jars. Mason's tools. 
. 70 Smith shop tools, track tools, cupping tools, hot sets. 

Set screws. 
0.60 Hot work and battering tools generally. Bolt and rivet 
headers. 

Hot drop forging dies. Rivet sets. Flatteners, fullers, 
wedges. 
0.50 Machinery parts. Track bolt dies where water is con- 
tinually running on dies (hot work). 

Navy Specifications. — The United States Navy specifies the 
following straight carbon tool steel for its general requirements: 



414 



STEEL AND ITS HEAT TREATMENT 



Class. 


I. 


II. 


III. 


IV. 


Carbon 


1.25-1.15 

0.35-0.15 

0.015-0 

0.02-0 

0.40-0.10 


1.15-1.05 

0.35-0.15 

0.015-0 

0.02-0 

0.40-0.10 


0.95-0.85 

0.35-0.15 

0.02-0 

0.02-0 

0.40-0.10 


. 85-0 . 75 


Manganese 

Phosphorus 

Sulphur 


0.35-0.15 
0.02-0 
. 025-0 


Silicon 


0.40-0.10 



Chromium and vanadium optional. 



Class I. Lathe and planer tools, drills, taps, reamers, screw-cutting 

dies; taps and tools requiring keen cutting edge combined with 

great hardness. 
Class II. Milling cutters, mandrels, trimmer dies, threading dies, 

and general machine-shop tools requiring keen cutting edge 

combined with hardness. 
Class III. Pneumatic chisels, punches, shear-blades, etc., and in 

general tools requiring hard surface with considerable tenacity. 
Class IV. Rivet sets, hammers, cupping tools, smith tools, hot-drop 

forge dies, etc.; tools requiring great toughness combined with 

necessary hardness. 

The Navy Department also maintains the requirements as to 
grade by requiring a steel which will stand rehardening a specified 
number of times without cracking. 

General Properties. — The following table shows the relative 
toughness and hardness of tool steel of the different carbon contents : 

Carbon, 
Per Cent. 

. 50 Toughness only. 

. 60 Great toughness with properties suitable for hardening and 

tempering. 
0.70 Excellent toughness, but with cutting edge. 
0.80 Tough tool steel, withstanding shocks, etc. 
0.90 Good cutting edge but with toughness an important 

factor. 

1 . 00 Toughness and cutting edge about equal. 

1.20 Great hardness combined with some toughness. 

1 . 30 Great hardness in cutting edge. Toughness slight factor. 

1.40 Extreme hardness in cutting edge first requirement. 

Toughness slight factor. 
Some metallurgists consider that it is safer to select a too hard steel 
and draw the temper at a higher temperature than to choose a too 



TOOL STEEL AND TOOLS 



415 



soft steel with a view to increasing its hardness by a weaker temper- 
ing. Opposed to this is the fact that the higher the carbon 
content the more the care which will be required in the harden- 
ing operation, since the steel becomes more sensitive to 
overheating. 

GENERAL TEMPERING COLORS FOR TOOLS 

Faint yellow: Steel-engraving tools. 

Light turning tools. 

Hammer faces. 

Planing tools for steel. 

Ivory-cutting tools. 

Planing tools for iron. 

Paper-cutting knives. 

Wood-engraving tools. 
Light yellow: Milling and other circular cutters for metal. 

Bone-cutting tools. 

Scrapers for brass. 

Shear blades in general. 

Boring cutters. 

Leather-cutting dies. 

Screw dies. 

Inserted saw teeth. 

Taps. 

Rock drills. 

Chasing tools. 

Penknives. 
Straw: Dies and punches in general. 

Moulding and planing cutters for hardwoocL 

Reamers. 

Gouges. 

Brace bits. 

Plane irons. 

Stone-cutting tools. 
Deep straw: Twist drills. 

Cup tools. 

Wood borers. 

Circular saws for cold metal. 

Cooper's tools. 

Augers. 



416 



STEEL AND ITS HEAT TREATMENT 



Brown. Drifts. 

Circular cutters for wood. 

Dental and surgical instruments. 

Axes and adzes. 

Saws for bone and ivory. 
Peacock: Cold sets for steel and cast iron. 

Hand chisels for steel and iron. 

Boiler-maker's tools. 

Firmer chisels. 

Hack saws. 
Purple. Moulding and planer cutters for soft wood. 

Smith tools and battering tools generally. 
Blue: Screwdrivers. 

Saws for wood. 

Springs in general. 
These colors are for general crucible steel with low manganese. 
Their applicability to particular work and special steels may be taken 



Fahr. 

Ill 












































a> 

|l400 

A 






























1380 
1360 































% 



M 



Yz 



% 



Diameter in Inches 
Fig. 261. — Temperature-size Curve for Hardening Tools. 



in a general way, but that temperature must be adopted which will 
suit the special work or steel in hand. 

Hardening. — We have previously discussed the fact that with any 
increase in the mass of the steel there is a corresponding decrease 
in both the maximum surface hardness and the depth of hardness, 
when quenched from the same temperature. This difference in 
hardness is due to the difference in the rate of cooling of the small 



TOOL STEEL AND TOOLS 



417 



and large sections. In order to produce the same degree of hard- 
ness in a small and large section, as applied to small tools, it is neces- 
sary to heat the larger section hotter for hardening than the smaller. 
To illustrate: Matthews and Stagg have worked out the relation of 
mass to temperature for one particular grade of the same tool steel 
in which the sizes varied from ^ in. diameter to f in. diameter, and 



no 

105 

100 
95 







































III 






Ha 


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lg Po 


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1200 1300 1400 1500 1600 

Fig. 262. — Loss of Hardness Due to High Hardening 



90 
1.00 

1.10 
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1.30 

1.35 
1.40 

1.50 
1.65 

1.70 
1.75 



1700 1800 

Temperatures. (Shore.) 



found that a difference of about 60° F. in heating was necessary to 
produce the same degree of hardness in the two extreme sizes. Their 
temperature-size curve is given in Fig. 261. 

The table on page 419 gives the approximate temperatures for 
handling general tool steels. Two columns are given under harden- 
ing temperatures as representing the best practice of two well-known 



418 STEEL AND ITS HEAT TREATMENT 

steel companies. As a general proposition, the lowest temperature 
should be used for hardening which will give the desired results: 
the use of abnormally high temperatures will increase the grain size, 
weaken the steel, and reduce the hardness. These last factors 
become even the more apparent with increase in the carbon content, 
as is roughly illustrated by the scleroscope readings as given in 
Shore's chart in Fig. 262. 

On the other hand, on account of mass action and other individual 
and distinctive shop conditions, it is difficult to set the upper limit 
over which hardening should not be done. Certain classes of work 
often require temperatures which might prove excessive for other 
work; thus one instance has come to the author's attention in which 
the hardening of certain 1 i^-in. rounds of 0.9 per cent, carbon stock 
are hardened at 1600° to 1620° F. and 80 per cent, more service is 
being obtained than from the same steel hardened at 1460° F. 
Again, another well-known company hardens 0.9 per cent, carbon 
steel of approximately the same size at 1370° F. and obtains better 
service than when hardened at higher temperatures. Each case, 
in other words, must be handled separately and those temperatures 
worked out which will give the best solution of that particular 
problem. 

Distortion Factors. — Slender pieces of steel, when hot, will bend 
under the application of a steady, even though slight, load. The 
weight of the part being heated for hardening is often sufficient to 
cause noticeable distortion if the tool is placed in the furnace in 
such a manner that it is not carefully supported. For this reason, 
such tools are best heated when held in a vertical position, with the 
point of support at the upper end of the piece, the tool being so held 
that it automatically comes to the normal position as will a plumb- 
bob. 

Distortion may be due to the initial condition of the steel, such 
as may result from forging, rolling, machining, etc. Any strains 
which exist in the tool previous to heating for hardening are relieved 
when the piece is heated, but the readjustment of such strains may 
cause a bending or twisting of the tool. In making the tool it is 
advisable to rough down to within about ys m - °f the finished size 
and then anneal in some non-oxidizing material to relieve the 
machining strains. If the tools are not straight after annealing, 
they should be heated, straightened while hot (do not straighten in 
the cold), and then reannealed. The tools are then finished and 
are ready for hardening. 



TOOL STEEL AND TOOLS 



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420 STEEL AND ITS HEAT TREATMENT 

The influence of annealing or previous hardening operations 
upon the change in shape, such as increase or decrease in diameter 
and length, or combinations of these, is very difficult to foretell. 
As a general rule, however, such changes are the more marked with 
repetitive hardenings, and with increased percentages of carbon. 

Changes in Length. — Most tool steel has a tendency to contract 
in length upon hardening, and especially after previous annealing 
or hardenings. This change, whether it is expansion or shrinkage, is 
dependent upon the chemical composition and uniformity of the 
steel, the grain size and influence of the mechanical elaboration 
and annealing, the uniformity and amount of heat used in hardening, 
the method and rapidity of cooling, and innumerable other variables. 
The commercial application of heat-treatment principles when ap- 
plied to fine tools for their standardization to exact measurements 
must be along the lines of standardizing each step of the process. 
The steel must initially be kept uniform in composition and physical 
test; the limits of treatment must be held to a minimum difference:' 
and having somewhat accomplished these aims, the average change 
of size for specific dimensions must be studied and the tool made 
accordingly. Thus in the matter of taps, by obtaining the average 
contraction lengthwise for a given size tap blank, under standard 
conditions, the thread may be cut upon lathes having the lead screws 
so adjusted that the pitch given to the tap before hardening will 
just come right after hardening. 

Changes in Diameter. — Assuming that the many other variables 
affecting distortion might be reduced to a constant or standard, it 
is a generally accepted condition that there is a relationship existing 
between the amount of distortion by swelling after hardening and 
the original diameter of the piece. Most slender tools, in ordinary 
commercial practice, have a tendency to expand in diameter after 
hardening. In factories where a standard steel and standard methods 
are in vogue the amount of increase is a very important factor, as 
is shown by the following data obtained from an exhaustive study l 
of taps : 

Thus this particular problem was attacked with the view of 
obtaining, under standard shop conditions, the average increase in 
diameter due to hardening for each size of tap. With these results 
it was then possible to determine the necessary angle diameter of 
the tap before hardening, so that the hardened tap would meet the 
final requirements. 

1 Woodward. 



TOOL STEEL AND TOOLS 



421 



Diameter of 


Average Increase 


Diameter of 


Average Increase 


Tap. 


in Diameter. 


Tap. 


in Diameter. 


Ye inch 




1| inch 


0.0025 


i 

8 


. 00025 


If 


0.0025 


1 
4 


0.0005 


2 


0.003 


1 
2 


0.001 


24 


0.003 


3 

4 


0.0015 


3 


0.0035 


1 


0.002 


31 


0.0035 


ii 


0.002 


4 


0.004 



It was also found, however, that experiments upon the same steel, 
under apparently the same conditions, showed that there may be 
very great variations in the effect of hardening upon the diameter; 
and when the various other factors are taken into account, the 
difficulty of prognosticating exactly what is going to happen is even 
more apparent. 

Heating. — Much of the difficulty experienced through distortion 
or cracking may be largely diminished by the proper application of 
the principles of heating such as have been discussed elsewhere. 
The heating should be done slowly, carefully and at a uniform rate. 
In no case should the temperature of the furnace be greater than 
the maximum temperature to which the steel is to be heated. After 
the steel has been thoroughly heated, a further continuance will 
only tend to weaken the steel by increasing the grain size. The 
furnace should be of such design, construction and operation that 
it shall be of uniform temperature over its whole hearth, shall heat 
at a uniform rate, shall not be greatly affected by the introduction 
of fresh charges, shall have a neutral or reducing atmosphere, and 
shall be under exact control. Further, it is not only the temperature 
to which the steel has been heated in the furnace that counts, but 
also the temperature (and uniformity of temperature) of the steel 
when it goes into the quenching bath. 

HEAT TREATMENT OF TOOLS 



In the following pages there is given a description of practical 
methods of treating certain tools. It is not intended that the data 
shall be comprehensive of all methods or of all tools, but shall give 
practical hints and methods which may aid the man in the small 
shop who may have such work to do but occasionally. It should 
also be remembered that the ideas given are representative of a 



422 STEEL AND ITS HEAT TREATMENT 

type of treatment or tool, and which may find application for many 
uses not given. The methods given are taken from practical work, 
and have given satisfaction. 

Chisels. — Chisels belong to that class of tools which can be 
advantageously hardened without first grinding away the " skin." 
Chisels are not hardened by heating up the whole tool, but only 
applying the heat to 2 ins. or so of the cutting end; too short a 
distance is detrimental, as the long, unhardened shank may bend 
under heavy blows. In quenching, care should be used to avoid a 
distinct line of demarkation between the hardened and unhardened 
parts, as otherwise the end may break off in service. After harden- 
ing the cutting end by immersing in water and at the same time mov- 
ing the chisel up and down, the tool should be removed while suffi- 
cient heat for tempering still remains in the shank. The cutting 
edge is brightened with emery cloth. Temper colors will soon 
appear in the brightened spot, which are caused by the heat from 
the shank " running down" towards the cutting edge; the temper 
colors may be " spread out " by holding the end near the fire if it 
is desired to have a broad band near the edge. When the proper 
temper color is obtained at and near the edge, the chisel should 
be immediately plunged into water until cold in order to prevent 
further softening. 

The degree of tempering depends upon the steel which has been 
used, the duties in service and upon the general experience and judg- 
ment of the hardener. In general it may be said that chisels for 
metal should be tempered to about a peacock color; for stone to a 
purple color, although many stone and granite chisels are drawn to 
only a light straw color; and for soft material to a blue color. If 
a very tough steel is desired, the chisels may be given a double 
tempering by heating to the temper color twice, the first color 
being rubbed off after quenching. The lower the carbon used the 
lower should be the tempering; in fact, a 0.45 per cent, carbon steel 
is sometimes used after hardening without subsequent tempering. 

An excellent plan in treating chisels is to oil treat the blank 
before forging out, quenching the steel in oil and from a temperature 
sufficient to harden it well. This will stiffen the shank and keep the 
head from upsetting during use. 

The problem of producing a good chipping chisel is simple, 
and yet has met with more difficulties than the majority of other 
such tools. The correct adjustment of temperatures plays a most 
important part in this work, For forging, the steel should be heated 



TOOL STEEL AND TOOLS 423 

up gradually and not much beyond a bright-red heat. 1 The work 
should be done rapidly, aiming to obtain the greatest reduction to 
size while the steel is yet near its first heat; as the temperature falls, 
the blows should be lighter and quicker. It has been the author's 
experience that added toughness is given to the steel if these light, 
quick blows are carried on until the steel nears a dull-red heat. If 
reheating is necessary, adopt the same care in raising the temper- 
ature as before. The chisel should be immediately hardened and 
tempered after forging when possible. If the pressure of work 
will not permit of this, the chisel should be allowed to cool as slowly 
as possible by sticking the hot end in dry dirt; this latter will tend to 
eliminate the cooling strains which might otherwise be set up. For 
hardening heat slowly to the lowest temperature at which the steel 
will harden (generally about 1350° to 1400° F.), allow the heat to 
penetrate and harden as usual. If care is used, and sufficient 
thought has been given to the design of the chisel, a hundred per 
cent, improvement may be easily obtained over the ordinary " hit 
or miss " method. 

An open-hearth steel which has given unusual service for pneu- 
matic chipping chisels is as follows: Carbon, 0.90 to 1.00 per. cent.; 
manganese .50; phosphorus and sulphur low; chrome, 0.50 per 
cent. The hardening temperature of this steel is about 1400° F. 

Die Blocks. — The problem of satisfactorily treating die blocks is 
one which will try the hardener's knowledge and patience, increas- 
ing in difficulty with the intricacy of design and size. Primarily 
the requirements of any die are (1) a hard face, (2) sufficient depth of 
hardening to prevent the impression from sinking, and (3) a tough 
back or body to take up the shock or blow. Further, the hardening 
must be conducted in such a manner as will prevent any change of 
size — such as warping — besides producing a clean, sharp impression 
free from scale, pitting or checks. These fundamental requirements, 
assuming that the right kind of steel is used, involve the factors of 
proper heating and the most suitable method of cooling. 

1 For the convenience of the tool hardener who has to work without a pyrom- 
eter and must gauge temperatures by the eye, the following table of approxi- 
mate heat colors in moderate diffused daylight is given: 

White 2200° F. Cherry or full red 1375° F. 

Light yellow 1975 Medium cherry 1250 

Lemon '. 1825 Dark cherry 1175 

Orange 1725 Blood red 1050 

Salmon 1650 Faint red 900 

Bright red 1550 



424 STEEL AND ITS HEAT TREATMENT 

More die blocks are warped or cracked through improper heat- 
ing than through any other cause. It is absolutely essential that 
the entire mass of the steel of the block shall be heated carefully, 
uniformly, through and through, to the proper temperature. 

It is always best to allow the die to heat up with the furnace so 
that any strains which may exist in the steel may be removed grad- 
ually and give the mass of steel ample time to adjust itself to the rise 
in temperature. If the furnace for heating for hardening is already 
at or near the quenching temperature when the die is ready for 
heating, it will be advisable carefully to preheat the die in a separate 
furnace. Equipment is subsequently described for automatically 
preheating and full-heating in the same furnace. In no case should 
the temperature of the heating furnace be greater than the maximum 
temperature from which the steel is to be quenched. The practice 
— unfortunately common — of forcing the furnace in order more 
quickly to heat the steel is to be strongly condemned ; such practice 
must inevitably result in the overheating of the edges or corners 
of the die and produce unsatisfactory results. The extra time 
spent in careful and uniform preheating, and in all subsequent heat- 
ing operations, will be well worth the expense. This point is con- 
sidered further on page 124. 

The exact temperature best suited for hardening may be men- 
tioned here only in a general way. The chemical composition of the 
steel, the size of the die block, the depth of hardness required, the 
condition of the steel before hardening, and many other factors 
must be taken into consideration. In general, nickel and chrome 
nickel steels may be quenched at lower temperatures than those used 
for the corresponding carbon steels, while vanadium and chrome 
vanadium steels usually require higher temperatures. Again, the 
greater the mass of the steel, and the greater the depth of hardness 
required, the higher is the temperature for quenching. Some die 
hardeners even find that a temperature which will produce a slightly 
coarse grain is advisable for certain classes of work, such as dies 
for cold forming under a heavy drop. In other words, the proper 
temperature for hardening must be determined by 'experiment, but 
the lowest temperature which will produce the desired results is 
always the best. 

The next, and a vastly important factor, is the duration of heat- 
ing at the predetermined temperature for hardening. The point 
to be emphasized is that the mass of the steel must be uniformly 
heated throughout — and this takes time. Not only must the outer 



TOOL STEEL AND TOOLS 425 

sections attain the necessary degree of heat, but also the very- 
center of the die. Disregard of this fundamental principle is the 
basis of a large proportion of the failures through cracking or warp- 
ing, and which are so often attributed to " the steel is no good." 
If the die block lays directly on the hearth of the furnace (and this, 
it might be mentioned, is not the best practice, since any work is 
best heated when the hot gas currents can circulate entirely around 
it), the penetration of the heat may be roughly determined by 
moving the block to one side and noting the color of the space va- 
cated ; if this area is not of the same color as the rest of the furnace 
floor the heat has not thoroughly penetrated to the center of the 
die. This test, called the " drawrouagh " by old English hardeners, 
should be followed even though the play of heat colors over the ex- 
posed portions of the block appear uniform. 

For protection of the impression from oxidation through contact 
with the air, the faces of the dies may be packed in carbonaceous 
material. One of the large manufacturers of silverware packs his 
dies as follows: A small sheet-iron pan, about 2 ins. high and 
about an inch or so wider than the die all around, is partly filled 
with granulated animal charcoal or bone. The die is then pressed 
firmly upon the charcoal, forming an impression, and is then care- 
fully removed. This impression is sprinkled with powdered animal 
charcoal, and with very fine steel filings. The die is carefully 
replaced in position, surrounded with more granulated animal char- 
coal to the height of the pan, and the space between the top of the 
pan and the die carefully luted with fire-clay. Upon heating, the 
filings and powdered charcoal fuse together upon the surface of the 
steel, forming a protective coating which eliminates oxidation during 
heating, but which is washed off during the quenching. A little 
brushing with oil and emery powder will immediately produce a 
clean, bright surface. Another method is first to paint the surface 
of the die with a thick paste made of linseed or cottonseed oil and 
powdered bone-black; the die is then placed in a shallow pan upon 
a half-and-half mixture of fresh bone and powdered charcoal, in a 
suitable pan or box, which is then filled and luted as above. 

The old-time method of an immediate and total quenching of 
the block until it is quite cold should be attempted only with the 
simplest forms and small sizes of dies. Large blocks have a great 
tendency to warp, bulge, or even crack if a total immersion is adopted, 
this being caused by the unequal contraction of the metal of the 
surface and of the core. It may be said, however, that this difficulty 



426 STEEL AND ITS HEAT TREATMENT 

may be largely avoided if the block is previously given. a special 
treatment consisting of oil quenching from just over the critical 
range, followed by an annealing at a temperature just under the 
lower critical range. Total quenching also should not be used if an 
extremely hard face is desired, since the heat cannot usually be 
removed quickly enough. 

The best practice for hardening large die blocks consists in first 
carefully preheating the die, then slowly raising it to the hardening 
temperature and allowing it to soak at this temperature until it is 
thoroughly heated. For this work a furnace should be used in which 
accurate temperatures and uniform heating can be obtained. Large 
blocks may be most easily handled by the use of a hoist on a swinging 
run-way, mono-rail or overhead crane, and equipped with tongs 
or " dogs." The dogs fit into holes which have previously been 
drilled in opposite sides of the block about half way between the 
upper and lower faces. ' 

When the block is properly heated, it is removed to the front of 
the furnace, gripped with the dogs, run over to a position above the 
quenching tank, lowered face-downwards entirely into the water or 
oil for a few seconds (to prevent warping), and then raised out of 
the quenching bath until immersed about 1 in. deeper than the 
depth of the deepest impression in the die. The surface of the bath 
should be kept in motion, or else the block should be slowly raised 
and lowered a little so that there will be no one line of hardening. 
The hardening will be greatly increased if a stream or heavy spray 
of water (assuming water to be used for hardening) is directed 
against the face of the block, or into the impression. In the case of 
blocks which contain a deep impression, such as are used for certain 
classes of gears, etc., it will be necessary to have a stream of water 
thus impinge upon the impression in order to harden it; the face 
of the block will take on great hardness, and the heat from the 
unsubmerged part will gradually be drawn out. 

When the face of the block is entirely cold, and the majority 
of the heat taken out of the other portion of the block (usually at 
about a very dark red, but dependent upon the size of the block), 
it is raised out of the water, reversed to face up, and brightened with 
emery paper. The heat in the hot part of the block will gradually 
temper the hardened face. When this approaches a good straw color 
the block is immersed in water or oil until cold; in some instances 
where a softer block is desired, the block may be allowed to cool in 
the open without the use of water to stop the temper. In case there 



TOOL STEEL AND TOOLS 



427 



is not sufficient heat left in the block after hardening to bring out 
the desired temper color, the block may be stood in front of the fur- 
nace, back to the heat, or placed on a hot bar of steel, or laid on top 
of a low smith fire, until the proper temper is reached. Die blocks 
will generally give more uniform service when drawn in a tempering 
bath when such is possible. In case the block is of intricate design 
and requires very particular tempering in weak spots, this may be 
done by the local application of heat by means of hot plates, etc. 




Fig. 263. — Intake End of Special Furnace for Hardening Forging Dies. 
(Lake, in " Machinery.") 

Die blocks hardened and tempered as directed above should pro- 
duce a strong, tough base and core, increasing in hardness as the 
face is approached. 

Continuous furnaces for heating die blocks for hardening, in 
which the preheating as well as the final heating is done in the 
same furnace, are illustrated in Figs. 263 and 264. Four runways, 
filled with 3-in. malleable-iron balls, extend throughout the length 
of the heating floor of the furnace. Castings which fit over the 



428 



STEEL AND ITS HEAT TREATMENT 



balls in two of these run-ways and which are of suitable size to carry 
one of the dies, are placed in position in the end of the furnace shown 
in Fig. 263. When the cold die is placed on this casting, as shown at 
0, one of the pneumatic pushers P is brought into play and the cast- 
ings act as a cart, carrying the die into the furnace. By following 
the first casting and its die with others, the furnace is gradually 
filled, the ram pushing the whole line of dies further into the furnace 
with each new addition. The furnace is so designed and operated 
that the temperature at the charging end is low, but gradually 




Fig. 264. — Quenching and Tempering Dies. (Lake, in " Machinery.") 



increases up to the maximum near the other end of the furnace; 
preheating and the final heating are thus obtained in the same 
furnace. Each furnace is double tracked and heats two rows of 
dies at once. At the end of the furnace shown in Fig. 264 the 
hot dies come out on the extension of the run-way marked Q. The 
faces of the dies are turned downwards so that the dies may be 
picked up by the traveling crane and lowered into the quenching 
tank, as shown at R. A stream of water also plays against the 
impression, as usual. The hot plate shown at T is used for the 
tempering. 



TOOL STEEL AND TOOLS 



429 



Engraved dies for spoons, forks, knives, etc., are treated at one 
plant by the following method. After packing and heating as 
described in a previous section, the dies are quenched face up in 
water at a temperature of about 70° to 80° F., to a depth of within 
about | in. of the face. Water at this temperature seems to give the 
best results in this particular instance — colder water is too harsh, 
while warmer water does not sufficiently distribute the strains nor 
give sufficient hardness. As soon as the cooling effect just begins 
to creep towards the face of the die, and which only takes a few 




Fig. 265. — Method of Hardening Engraved Die. 

seconds, the die is immediately wholly immersed in a vertical posi- 
tion in the water, with the impression turned toward a heavy stream 
of water which impinges directly upon it. The arrangement of the 
quenching bath is shown in Fig. 265: the die (a) rests upon a wire 
platform (6); the water is supplied under pressure through a l|-in. 
pipe (c), flowing out through a j-in. slot (d) which extends from the 
level of the die support to the top of the pipe. The die remains in 
the water bath until the " singing " has stopped, about 50 to 90 
seconds, and is then cooled in oil until cold. The hardened die is 
later tempered in oil to 435° F. 



430 STEEL AND ITS HEAT TREATMENT 

Many alloy steels have been experimented with in recent years 
for the purpose of increasing the production of forgings from a 
given impression, thus avoiding the loss of time and expense incurred 
in redressing the die-blocks. A chromium nickel steel containing 
about 0.50 to 0.60 per cent, carbon, 0.50 per cent, chromium and 1.50 
per cent, nickel has been found to give most economical results. 
These die blocks are hardened and tempered in the usual way, 
using a temperature of 1400° for the hardening heat. If the carbon 
content runs above 0.60 or 0.65 per cent, it has been the author's 
experience that cracking during or directly after hardening may 
result. These blocks are greatly improved, not only in the length 
of service to be obtained, but also in the ehmination of warpage 
during hardening, and of danger of cracking, by giving the block, 
before machining, a full heat treatment and toughening or annealing; 
blocks which approximate the composition noted should be quenched 
in oil from a temperature of 1400° to 1450° F., and then full annealed 
at about 1250° F. Such a treatment gives excellent results, and will 
also show up any defects such as pipes, seams, etc., before the expen- 
sive machine work has been done. 

Dies used in engraving work, and in the jewelry and optical 
crades, must have a glass finish, both in smoothness and in hardness. 
If subjected to the usual quenching, followed by sand-blast, acid 
bath or cyanide, a large amount of stoning and polishing would be 
required. This may be obviated by the use of borax or boracic acid 
in the following manner. Fill the matrix with powdered boracic 
acid and place near a fire until it melts, which temperature is con- 
siderably below the tempering point or color of the steel. Follow 
this with a second addition of boracic acid and then harden as usual. 
Although the salt will generally come off in the quenching, it pro- 
tects the polished surface of the die and does not interfere with the 
hardening. In case the salt does not come off in quenching, it may 
be easily removed by live steam or boiling water. The hardening 
may be done by complete or partial submersion, depending upon the 
thickness and general design of the die. Engraving dies are usually 
tempered to a light straw color. 

Drills. — For occasional work in hardening drills, the following 
procedure may be used: If an open fire is the only available source 
of heat for hardening, the points of the drill should be kept out of 
the hottest part of the fire at first, drawing them in as the upper 
parts become heated. The heat should extend over a considerable 
portion of the drill. Quench vertically in water, and keep the drill 



TOOL STEEL AND TOOLS 431 

moving up and down so that there is no abrupt line of demarkation 
of the hardening. If the drill is held quietly in the water, fracture 
across the water line is a common occurrence when the drill is placed 
in service. Allow the drill to remain in the water until the im- 
mersed part is entirely cold. Remove, brighten, and allow the heat 
in the shank to run into the hardened part until a dark straw color 
appears on the cutting edge. The drill should then be immediately 
and entirely immersed in water. If there is not sufficient heat in 
the shank to bring out the temper color, use hot ashes, or similar 
means. The drawing operation upon hardened drills should pref- 
erably be carried out in an oil or salt bath subsequent to straight- 
ening; drawing expensive tools to color is poor practice. 

It is always advisable, however, if an open fire must be used for 
heating, as noted above, to heat the drill in a pipe or tube to prevent 
the direct contact of the fire and the steel, or with charcoal to prevent 
oxidation. The heating should be done slowly, uniformly, and to as 
low a temperature as is possible and consistent with the desired 
results. 

In cases where a large number of drills are to be hardened, it 
is advisable to use a special hardening tank. The shape of the 
lands of the drill is such that the steam formed by the contact of 
the water and the hot metal will in many instances prevent the 
water from penetrating to the flutes and properly hardening them, 
besides having a similar influence on the end of the drill, which will 
become the new cutting edge as the point is ground back. This 
buffer or blanket of steam may be eliminated by maintaining a 
constant flow of cold water into the grooves and against the end 
of the drills. Perforated pipes may be placed up the sides of the 
quenching tank, and through which the cold water is forced into the 
grooves; similarly, a jet from the bottom strikes against the end of 
the drill. 

For drills for holes under \ in. in diameter, the hardening heat 
should be allowed to penetrate only through the cutting part. The 
drill should then be quenched entirely and the temper drawn to suit 
the work. The reason for not allowing the hardening heat thor- 
oughly to penetrate to the core of the drill is that sudden quenching 
of a small, slender piece might cause severe strains to be set up in the 
steel; such drills also require a tough core to be able to withstand 
the torsional effect in the actual drilling operation. Most of the small 
drills are quenched in oil. The temper color is usually a dark straw. 
If the tempering is accomplished by placing the drills upon a heated 



432 STEEL AND ITS HEAT TREATMENT 

bar, the cutting parts must be allowed to project for some distance 
over the edge of the hot bar, for otherwise the heat will be too sud- 
denly applied. 

Milling Cutters. — Under this class are included cutters of varying 
description, such as milling cutters, forming cutters, slotting cut- 
ters, angle cutters, etc. This consists, in general, of a cylindrical 
piece of steel with a bore through the center, and teeth on the cir- 
cumference, sides, or both. The unequal forces of contraction 
and expansion affect these tools to a large extent. In designing 
a cutter, as large a mandrel hole as is possible should be used, as 
larger holes will permit the steel to be hardened more uniformly. 
If the mandrel holes are standardized, large cutters may have a 
part of the sides (in the absence of side or angular teeth) dished or 
paneled out at the place which would tend to harden last, that is, 
half way between the two circumferences. 

Great care should be used in heating milling cutters for harden- 
ing. The heating atmosphere should be neutral or slightly reducing 
to protect the teeth. If an open fire is used, the fuel should not be 
allowed to come in contact with the cutter: this may be done 
by resting the cutter on a fire-brick or plate. If a hearth furnace 
is used, the cutter should not touch the floor or walls of the furnace, 
but should be supported by fire-bricks or other suitable methods. 
If tongs are used in handling, care must be used so that the tongs 
do not touch the cutting edges; the use of wires is better practice. 
If the cutter is supported on bricks, or laid on plates, it must be 
turned repeatedly in heating so as not to leave any unevenly heated 
spots. The cutter may be conveniently held in the quenching bath 
by using a small round bar which has three prongs welded to one end, 
and which extend at right angles to the axis of the bar, by slipping 
the other end of the bar through the mandrel hole of the cutter; the 
latter will rest on the prongs, and then can be conveniently lowered 
into the quenching bath. 

Ordinary cutters are best hardened by the use of two small cir- 
cular plates of a diameter slightly greater than that of the cutter, 
and with holes bored through the center corresponding to the size 
of the mandrel hole of the cutter. One plate is placed on each end 
of the cutter, and the whole placed on the suspension tool as de- 
scribed above and immersed vertically in the quenching bath. 
By the use of these plates, the hardening will affect the steel along 
the entire length of the teeth and at right angles to the center line 
of the cutter. This will also eliminate the circular fracture or 



TOOL STEEL AND TOOLS 433 

flaking of the teeth which so often characterizes milling cutters 
subjected to uneven cooling. While in the quenching bath, the cut- 
ter should be moved up and down and not from side to side; this will 
permit the solution to pass through the center hole and give an evenly- 
hardened core. The combined use of water and oil (" broken hard- 
ening ") in the following manner is good for hardening for large 
cutters: quench in water until the " singing " caused by the water 
boiling on the hot steel has stopped, and then immerse in oil until 
cold; warm the cutter in boiling water to relieve the strains and 
temper when convenient. Pack-hardening is also used to some extent 
for milling cutters in order to prevent oxidation; in this case each 
piece should be quenched separately. Salt baths and lead baths are 
also used for heating. One of the main points to be observed in 
quenching milling cutters is that long cutters should be plunged 
vertically and thin ones edgewise. 

The tempering of milling cutters is often done by the insertion 
of a hot rod through the mandrel hole and revolving the cutter on 
it until the proper temper color is obtained. The most satisfactory 
results are to be obtained with the use of an oil bath, as an even hard- 
ness can be best obtained in this manner. Small cutters are tem- 
pered to a light straw color, or yellowish-white. For medium-sized 
cutters a good straw color may be used. Very large cutters, on 
account of the lesser effect of the hardening, may not require temper- 
ing, but it is always advisable to heat them in boiling water to make 
them uniform and remove the hardening strains. 

For hollow mills it is not necessary to heat for hardening very 
much above the teeth, as it is not required that the back should be 
hard. Harden with the teeth upwards, working the piece up and 
down in the quenching bath to get the solution circulating through 
the hole. 

T-slot milling cutters should be hardened, not only through the 
cutting portion, but also through the entire length of the neck, 
especially if this is of small diameter. In tempering, the cutting 
portion should be drawn to a straw color and the neck to a blue color. 

Files. — Before the file blanks can be ground and the teeth cut 
it is necessary to anneal the steel. This is often accomplished by 
packing the blanks in air-tight oblong boxes and annealing at about 
1300° to 1400° F. 

Lead baths continue to be most used as the heating medium. 
Salt baths have been tried with varying degrees of success, but in 
the main have proven unsatisfactory. This is due in a large measure 



434 STEEL AND ITS HEAT TREATMENT 

to the fact that oxide of iron (scale) may settle upon the teeth of the 
file, causing soft spots when hardened. The method of dipping the 
file into a solution of ferrocyanide and allowing the coating to dry 
upon the surface of the steel before heating has been tried. The 
objections to the use of this method are that a decomposition of the 
ferrocyanide will yield additional iron oxide and poisonous fumes. 
Other salts of a harmless character have been tried with little success. 
The general procedure is to cover the file with a paste which pro- 
tects the edges of the teeth in the hardening process, heating in lead 
to the proper temperature (about 1400° F.) and quenching in water 
in a vertical position. One file-maker uses a paste made of the 
following base: ground charred leather, 2 parts; table salt, 4 parts; 
and flour, 3 parts. The file is given a coat of this paste, which is 
allowed to dry before heating. It is said that the melting-point of 
this paste will give the proper hardening temperature. After being 
hardened, and while the file is still warm, it is put through the final 
straightening process. 

Half-round files require particular attention on account of their 
tendency to warp : before hardening, the file is bent back on a fixed 
template of such form as experience has shown will bring the file to a 
true line upon hardening; the file is placed again in the template 
before it is quite hard, strained to the proper degree, and water is 
thrown on the upper surface of the file to make it quite cold before 
the strain is relieved; the file is then entirely quenched and will 
usually return " to the true " after the final hardening. 

After the final straightening the files are " scrubbed " to remove 
the paste, and are then washed in lime water and dried by holding 
them in steam. The tang is then toughened or " blued " by dipping 
it into a special bath maintained at the proper temperature. 

File steel will vary in carbon from 0.90 to 1.60 per cent., accord- 
ing to the size, shape and use of the file; manganese under 0.40 per 
cent.; low phosphorus and sulphur; and in the case of exceptionally 
good files, a small percentage of chrome. Nickel is generally con- 
sidered as detrimental to files. 

Reichhelm 1 shows the detrimental effect of heat variations in 
hardening in the microscopic photographs of two fractures of the 
same file magnified 160 times. This file is one of the highest grade 
produced in Europe, and Fig. 266 shows the fracture of this file as 
imported, while Fig. 267 shows a fracture of the same file, a section 
of which was rehardened, after the exact degree of heat required 
1 " Machinery," Dec, 1914. 



TOOL STEEL AND TOOLS 



435 




Fig. 266. — Photomicrograph of High-grade Foreign File. X 160. 
(Reichhelm.) 




Fig. 267. — Photomicrograph of Same File Rehardened. X160. 
(Reichhelm.) 



436 STEEL AND ITS HEAT TREATMENT 

for this particular steel had been experimentally determined. Fig. 
266, therefore, shows the result of the best hardening practice in 
Europe, aided by the pyrometer, while Fig. 267 shows the hardening 
of this identical file by the correct heat automatically maintained. 
That any number of files, or tools of any kind, can be hardened so 
as to show uniformly the excellent fracture exhibited in Fig. 266 is 
due to automatic heat control, as has been demonstrated conclusively 
in daily practice for over three years. 

Both of the photographs of fractures have been pronounced 
excellent by competent judges, but the decidedly finer grain and 
more even diffusion of the carbon shown in Fig. 267 produced a 
difference in the durability of the file teeth of nearly 50 per cent., as 
compared with the section of the file as originally hardened and shown 
in Fig. 266. 

Punches and Dies. — Similar to all round tools, punches show a 
great tendency to flake off at the corners, sometimes a whole ring 
breaking off. Assuming proper heating, this may be overcome to 
a large extent by means of a water spray. Dies of intricate shape 
and possessing sharp angles should be most carefully handled. It 
is often advisable to fill these angles with a little putty or fire-clay 
to lessen the hardening effect and prevent the formation of quench- 
ing strains at right angles to the diagonal. A piece of binding wire 
may also serve for this purpose. Dies should generally be quenched 
flat, depending upon the shape of the piece. Small punches should 
not be quenched in real cold water on account of the liability to 
cracking under sudden cooling — an oil bath or lukewarm water 
is far preferable. Dies or any press tools having holes near the 
edge should always have these holes filled with clay in order to pre- 
vent cracking or too great hardening; graphite or asbestos may also 
be used for plugging the holes for stripper or guide screws. Punches 
and dies are generally tempered to about a straw color, the depth of 
this varying according to the thickness and hardness of the material 
to be punched. The tempering may be carried out by setting the 
hardened pieces in front of a hot furnace, laying on hot plates, in 
oil baths or in hot sand. 

Reamers. — Reamers may be heated in lead to protect the cutting 
edge from the direct action of the heat and oxygen. The lead may 
be prevented from sticking to the tool if the latter is brushed, in 
the case of small reamers, with a little soft soap. Larger reamers 
may be protected with a paste made of black lead and water or 
lampblack and linseed oil, both of which should be allowed to dry 



TOOL STEEL AND TOOLS 437 

on the tool before heating for hardening. If the reamer has been 
hardened by the use of water alone, and is larger than f in. 
in diameter, it is advisable to hold it over the fire directly after 
being removed from the hardening bath, or to set it in hot water 
for a few moments, in order to remove — as far as possible — the 
strains which have been caused by the hardening process. This 
should always be done in the case of shell reamers and other special 
reamers of any considerable size, whether the quenching medium 
has been oil or water. Broken hardening is most excellent for tools 
of this description. Large fluted reamers require to have only the 
ribs heated to the proper temperature, and then quenched; temper- 
ing will not then be required. Ordinary fluted reamers are tempered 
to a yellowish white or very light straw color. Six-sided or eight- 
sided reamers may be tempered to a light straw color. Square 
reamers, triangular reamers and half-round reamers may be tem- 
pered to a dark straw color, due to the fact that they take hold of 
the work more deeply and might break if not tempered a trifle 
softer. 

Half-round reamers should not be quenched vertically, but with 
the half-round side at an angle of 20 to 45 degrees to the surface of 
the bath. If half-round reamers should be quenched vertically, 
it will be necessary to move them in a horizontal manner in the 
direction of the half-round side at the same time as immersed ver- 
tically. 

The shanks of reamers, taps, drills, broaches and similar tools 
may be toughened by local lead tempering. 

Rings. — Rings, collars and hollow tools comprise a class which 
require great hardness in the inner circumference or bore. Quench- 
ing is usually done by means of allowing a full stream of water to 
flow through the bore if it is quite small, or in the case of tools with 
larger bores the insertion of a small pipe with a series of holes in its 
circumference and through which a continuous stream of water may 
be forced, forming a spray. In the first case it is advisable to set 
the tool upon an asbestos-covered washer in which has been cut a 
hole slightly larger than the size of the bore of the tool and then 
apply the flange end of the water-supply line or pipe to the other 
opening. Rings or collars requiring resistance to frictional wear 
require no tempering. Eccentric rings cannot be quenched as usual, 
as the relative thickness and thinness of the opposite sides would 
tend to give unequal expansion and contraction and cause the hole 
to become oval-shaped. This may be overcome by binding a small 



438 



STEEL AND ITS HEAT TREATMENT 



piece of iron or steel to the thin side, heating, and quenching ver- 
tically. 

Rivet Sets.— Rivet sets should never be quenched directly by 
immersion, as this will tend to make the edges of the cup break off, 
the center to remain soft, and leave a line of great weakness between 




Fig. 268. — Rough Method of Hardening a Rivet Set. 



the hardened and unhardened parts. A simple and proper method 
is to hold the cup under or over a stream of water so that the latter 
will impinge directly upon the bottom of the cup, as shown in Fig. 
268. If there are numbers of rivet sets to be hardened, an arrange- 
ment of clips or holders under each tap or spigot may easily be set 
up. The tempering may be carried out as in the case of chisels 
(permitting the heat in the shank to temper the cup) or the shank 



TOOL STEEL AND TOOLS 439 

may be placed in a lead bath and the color allowed to run up into 
the cup; the rivet set should then be entirely quenched to prevent 
further softening. 

Brearley makes the following points, which are of great interest. 
Rivet sets may have a short life due to the wear on the head, which 
is as often a failure as that produced by actual fracture. This is 
pronounced in the case of annealed stock. He advises hardening 
the head in oil before hardening the cup. Upon reheating for hard- 
ening the cup, and tempering, a steel of great toughness is obtained, 
which neither splits nor forms a mushroom head. 

Saws. — Saws may be hardened by either of two methods — direct 
immersion, or press or roll hardening. Circular saws may be heated 
by enclosing in a sheet-iron case or box between layers of charcoal. 
Sufficient space for expansion must be allowed to eliminate chance 
for buckling. Saws may also be heated on the hearth (if level) of 
any type of hardening furnace; it is advisable, however, to rest 
the saw on an iron or steel plate so that the heating may be gradual 
and uniform. The greater part of the secret for the successful hard- 
ening of saws without buckling is a slow and careful heating. The 
saws when heated to the proper temperature may be taken out 
separately with tongs or a J-shaped hook. For direct quenching 
they should be immersed edgewise and in a perfectly vertical position. 
It is better to have a thin layer of oil on the surface of the water 
bath, as the oil will ignite when the hot saw enters it, forming a 
thin, protective coating on the saw and thus lessening the risk of 
fracture. Oil alone, or oil with tallow dissolved in it will give suffi- 
cient hardness for thin saws. The saws may also be placed between 
lumps of tallow. The latter (tallow) is a better hardener than 
oil, and therefore gives a greater and deeper hardening. Thin cir- 
cular saws, and all ordinary saws such as hack saws, hand saws, etc., 
may be most satisfactorily hardened by means of a press. A com- 
mon and inexpensive method is to have two cast-iron plates hinged 
together, with the inner surfaces well oiled with a heavy oil. The 
hot saw is placed between the plates, which are then clamped to- 
gether and held until the saw is cold. Thin band saws are often 
hardened by means of rolls. Circular saws for metal cutting should 
be tempered to a dark purple color, or to a light blue for wood cutting. 
Hack saws require tempering to a light purple color. 



CHAPTER XX 

MISCELLANEOUS TREATMENTS 

The following examples and discussions of certain heat-treatment 
methods have been selected in an arbitrary manner as representative 
of distinct classes of work. Many others might just as well have been 
taken, but it is felt that those selected will perhaps illustrate in a 
general way some of the many problems which arise in the course of 
ordinary heat-treatment work. 

GEARS 

Gear-steel Classification.- — Automobile and similar machine 
gears may be broadly grouped according to the method of heat 
treatment, which, of course, is dependent upon the composition of 
the steel. Thus the three classes are: 

(1) The case-hardened gear, using a steel of low-carbon content 
— generally less than 0.25 per cent — and depending upon the case- 
carburizing process to give an outer layer of high-carbon steel and 
upon the subsequent hardening processes to produce the necessary 
wearing surface of sufficient hardness. 

(2) The oil-hardened and tempered gear, using a steel of the alloy 
type of about 0.45 to 0.55 per cent, carbon. 

(3) The hardened gear (without subsequent tempering), using a 
steel of an intermediary carbon content — about 0.30 per cent. 

Requirements of Gears. — All high-duty gears require that the 
steel shall be readily forgeable and machineable, and that after 
treatment it shall have the greatest possible hardness with the least 
possible brittleness. In this connection it may be s'aid that surface 
hardness is often more desirable than tensile strength, while the 
question of brittleness is very important on account of shocks. 

Case-hardened vs. Oil-tempered Gears. — The merits or 
demerits of each type depend largely upon the point of view and the 
personal experience of the user. Expert opinion may differ widely, 

440 



MISCELLANEOUS TREATMENTS 441 

as is shown by the following excerpts from addresses by two well- 
known metallurgists. One says: 1 

" Several years of observation and contact with the trade leads 
me to prefer the case-hardened gear. The result of direct tests 
upon thousands of gears of both types leads me to the following con- 
clusions: (1) The static strength of a case-hardened gear is equal 
to that of an oil-hardened gear, assuming in both cases that steel 
of the same class and approximate analysis has been used and that 
the respective heat treatments have been equally well and properly 
conducted. (2) Direct experiments proved that the case-hard- 
ened gear resists shock better than the oil tempered. (3) As regards 
resistance to wear the same type is incomparably better, although 
perhaps not as silent in action. 

" One of the leading makers of gears has proved this to his own 
satisfaction of late by an arrangement of shafts and gears whereby 
energy is transmitted through two case-hardened gears, in mesh 
with each other, to two oil-hardened gears. The gears are of the 
same size. The conditions of the test were severe. Five sets 
of the oil-hardened gears have already been worn out, while the 
original case-hardened gears are still in service and show the tools 
marks. 

" Upon the part of many there is a strong objection to case hard- 
ening. In nine cases out of ten this is doubtless due to the fact 
that the case-hardening operation has not been reduced to a science. 
The depth of case, the relation of case to core, the time and tem- 
perature to produce certain results and the exact control of these 
conditions, together with an accurate knowledge of the material to 
be treated, are factors that enter into successful case-hardening 
practice. Further points in favor of this method are easier machin- 
ing of the blanks, and at least equal static and dynamic properties 
with less chance of injury in hardening." 

Then here is the opposing argument: 2 "For machine tools, 
hardened high-carbon alloy steel gears appear to be preferable to 
case-hardened gears for a number of reasons : 

" 1. Physically they are stronger and tougher and should there- 
fore be better able to resist sudden impacts and extraordinary 
loads. They do not show by file and scleroscope test the same 

1 J. A. Matthews, " Alloy Steels for Motor Car Construction," Journ. Frank- 
lin Inst., May, 1909. 

2 From a paper by J. H. Parker, before National Machine Tool Builders' 
Assoc. 



442 STEEL AND ITS HEAT TREATMENT 

degree of hardness as case-hardened gears, but, nevertheless, with 
proper design, the dense-grained gear-tooth resists wear more satis- 
factorily, as was demonstrated recently by the examination of a 
motor-car transmission that had covered over 100,000 miles. The 
high-carbon steel gears in this car still showed the original tool marks. 
Not long ago a designer of machine tools commented on the ap- 
parent softness of some hardened high-carbon gears, but found after 
several months of hard service that they still showed tool marks, 
thus proving hardness ample for wear. 

"2. In service, especially for clash gears, the superiority of 
these gears is most marked. On the clashing faces, case-hardened 
gears are likely to have the hard case chipped off, thereby exposing 
the soft core to the impact of clashing. The hard chips fall into the 
gearing and may find their way into bearings, thus causing trouble. 
High-carbon steel gears with a uniform hardness throughout do not 
chip, nor do they ' dub over.' 

" 3. The heat treatment of high-carbon steel gears is much 
simpler than that required for proper case hardening. It is shorter, 
less costly and produces a more uniform product, and as the gear 
is heated but once for hardening, as compared with three times for 
case hardening, the finished gear is certain to be freer from warpage. 
The cost of proper case hardening is not generally appreciated, but 
it has been found that a case-hardening steel must cost three to 
four cents per pound less than a regular high-carbon hardening 
steel, if finished gears made from both materials are to cost the 
same. 

" With all heat-treated gears, little points in design are impor- 
tant. The gear-teeth should not be undercut, for if the section at 
the root-line is smaller than at the pitch-line, greater hardness and 
brittleness is produced where least desired. Great differences in 
section should be avoided wherever possible, so as to do away with 
excessive warpage. Sharp edges and angles, even in key- ways, are 
the cause of internal hardening strains which frequently result in 
failures; hence, wherever possible, a fillet should be used in place 
of a sharp angle." 

Case-hardened Gears — Treatment. — The steel for a case-hard- 
ened gear should be low in carbon, preferably under 0.25 per cent.; 
should be carburized so as to produce a case of a depth of about 
^j or jj inch and contain a maximum carbon concentration of 
about 0.9 per cent.; and should then be suitably heat treated. 
Since the principles of case hardening have been described elsewhere, 



MISCELLANEOUS TREATMENTS 443 

it will be necessary here only to outline the process, which is as 
follows : 

(Gear blank) . 

1. Anneal. 

2. Rough machine to approximate size 
(3. Light re-anneal.) 

4. Finishing machine. 

5. Carburize at about 1600°-1650° F. 

6. Cool slowly in carburizing box. 

7. Reheat and oil quench from 1550-1625° F. 

8. Reheat and oil quench from 1350-1425° F. 
(9. Temper, if desired, to not over 400° F.) 

The temperatures given are only approximate, depending upon the 
analysis of the steel, the mass of the steel, the results desired, and 
various other factors. Nos. 3 and 9 may be omitted if desired. 

Oil-hardened Gears — Treatment. — For the higher-carbon steels 
used for oil-hardened gears it is always advisable to give the gear 
blanks a preliminary treatment to develop the highest qualities of 
the alloy steels and the greatest uniformity in their physical 
properties. This treatment will also give the greatest " softness " 
of^which the steel is capable. This preliminary treatment (before 
machining) is: 

1. Quench in oil from about 150° to 200° F. over the criti- 

cal range. 

2. Quench in oil from about 50° F. over the critical range. 

3. Anneal at a temperature about 75° F. under the criti- 

cal range. 

If this preliminary treatment is not given, the gears blanks should 
be given a thorough annealing. The slight reanneal after rough 
machining and before the final cut is optional; it always helps, 
however. 

The final treatment consists in an oil-hardening and tempering 
process. For the majority of alloy steels this quenching is done 
from a temperature about 50° F. over the critical range; in the case 
of chrome vanadium steels, however, the best results are generally 
obtained by the use of a higher temperature. The temperatures 
generally used for the standard types of alloy steels for automobile 
gears, approximating 0.45 to 0.55 per cent, carbon, are about as 
follows : 



444 STEEL AND ITS HEAT TREATMENT 

Chromium nickel steels: 

1.5 per cent, nickel, 0.5 per cent, chromium, 1400° F. 
1.75 per cent, nickel, 1.0 per cent, chromium, 1425 
3.0 per cent, nickel, 0.75 per cent, chromium, 1375 
3.5 per cent, nickel, 1.5 per cent, chromium, 1400 

Nickel steels: 

3.5 per cent, nickel 1400° R 

5.0 per cent, nickel 1375 

Chromium vanadium steel: 

Type " D " (1.0 per cent, chromium, 0.8 per cent, 
manganese, 0.16 per cent, vanadium) ...... 1575° F. 

Silico-manganese steel : 

1.5 per cent, silicon, 0.7 per cent, manganese. . 1550° F. 

The usual precautions should be observed such as uniform and 
thorough heating, protection from oxidation, etc. Further, the 
gear should be quenched in the direction of its axis so that the oil 
can be made to circulate around the teeth, etc. The notes given 
under " Milling Cutters " * might also be of interest in their bearing 
upon gear treatment. 

The tempering is usually done at a temperature of 400° F. or 
upwards, depending upon the nature of the steel and upon the 
results desired. It should again be stated that a longer tempering 
at the lower temperature is preferable to a quicker and shorter tem- 
pering at a higher temperature. Thus, if a gear were to have the 
temper drawn quickly, the teeth, which should be the hardest, will 
be softer than the hub, which will remain brittle; with a longer 
heating at a lower temperature this will not be the case, since the 
whole gear will have responded throughout. Similarly, for these 
reasons, it is inadvisable to temper gears " by color," but to use an 
oil bath or a mixture of low melting-point salts. 

For gears made of alloy steel with only about 0.30 per cent. 
carbon the tempering operation is usually omitted. It is always 
best, however, to reheat the oil-quenched gears in boiling water for 
a short time in order to remove the hardening strains; such treat- 
ment will have little or no influence on the hardness and strength. 
The quenching temperature for such steels will of course be higher 
by some 50° or 75° than that given under the 0.45-0.55 per cent, 
carbon steels. 

1 Cf . Ch. XIX. 



MISCELLANEOUS TREATMENTS 445 

SPRINGS 

The usual analysis for carbon steel springs is approximately: 

Carbon . 90 to 1 . 10 per cent. 

Manganese under 0.40 

Phosphorus under . 04 

Sulphur under 0. 04 

Silicon up to . 25 

It is dangerous to allow the percentage of carbon to run up to 
1.25 per cent, (as is sometimes done), on account of the possibility 
of the formation of free cementite, which is an extremely brittle 
constituent. A crack might easily start in an area of cementite and 
when once started would follow through the cementite to the outer 
surface. Lower carbons would preclude the presence of free cement- 
ite. Finely divided cementite would also be less dangerous, 
and this could be obtained by hardening at a lower temperature 
(about 1400° F.), since crystallized and granular cementite can 
only be obtained by heating for a prolonged time at a high temper- 
ature. 

Aside from improper analysis, the majority of spring failures and 
troubles may be laid to abnormally high temperatures for heating 
for fitting followed directly by quenching from whatever temper- 
ature the steel may happen to be at; and then, as if this were not 
bad enough, to temper by " flashing." From general knowledge it 
appears that the maker of springs has not kept pace with improve- 
ments in spring steel and with the increased severity of the duty 
expected of springs. 

The old practice of high temperatures and of forming and 
hardening springs with a single heating cannot be persisted in if 
maximum quality and service are to be secured. The " practical " 
spring-fitter generally heats the steel to about as high a temperature 
as it will take without burning. Its effect upon the structure of 
of steel has been explained in preceding chapters, and also above in 
its relation to very high-carbon spring steel. 

But even assuming that the proper temperatures have been used 
in fitting, the time taken to go through the forming operation is 
sufficient to give the steel a chance to cool down to a temperature 
which will not give the most satisfactory results in hardening. 
The steel is not of uniform temperature over its length so that, if it 
be quenched directly after forming, it will probably lock up internal 



446 STEEL AND ITS HEAT TREATMENT 

strains of uncertain magnitude — to say nothing of the insufficient 
hardening if the temperature be under that of the critical range. 
In other words, the spring should be put back in the furnace again (it 
being generally preferable that the maximum temperature for forming 
shall be the same as that required for hardening) and reheated for 
a few minutes so that it will be heated uniformly throughout at the 
right temperature. If high temperatures have been used for form- 
ing it will be advisable to allow the steel to cool to a temperature 
under that of the Ar range before reheating for hardening; if this 
is not done the steel will retain the coarse grain-structure character- 
istic of the high heat for forming. If it is found that the steel 
departs from its shape at all during this reheating, it may be put 
through the rolls again previous to quenching, the time occupied 
being small compared with that for the original bending. The 
spring should then be quenched in some good, heavy tempering oil. 

For drawing the temper it is never advisable to use the process 
known as " flashing." The practice of replacing the steel, after 
quenching, in a high temperature furnace until the outside of the 
steel reaches the desired temperature is one which cannot be too 
strongly denounced, because of the impossibility of uniform treat- 
ment. No time is allowed for the heat to soak to the center, with 
the result that the hardness increases from the outside — a most 
undesirable condition. All spring steel should be drawn back in a 
suitable low-temperature furnace maintained at the proper temper- 
ature. The steel should be kept in the furnace for a time sufficient 
to allow of a uniform heating throughout. Lead baths and salt 
baths are also used considerably for this work. 

The proper temperatures for treating carbon spring steel have 
been given considerable attention by the American Society for 
Testing Materials. Their experiments were made with test speci- 
mens If by f by 14 ins. long with straight edges, and analyzing 
about 1.10 per cent, carbon. The results of these tests (1911) 
are given in the tables on page 447. 

It is apparent that at a quenching temperature of 1500° F. the 
maximum results are obtained with a drawing temperature of about 
600° F., while with a quenching temperature of 1650° F. the maxi- 
mum elastic limit was found with a drawing temperature of about 
800° F. In the former group, Series A, 1500°-600° F., it was 
found that the angle of bend at rupture showed an average of 
slightly over 59°, there being considerable variation between the 
specimens; while in the second group, Series B, 1650-800° F., the 



MISCELLANEOUS TREATMENTS 



447 



average angle was slightly over 103°, without any specimen going 
below 76°. These results are particularly interesting in view of the 
fact that the critical range of these steels is about 1350° F., and 
that one would naturally expect that a temperature of about 1400° 
F., i.e., slightly over the critical range, would give the best 
results. Whether or not such would show up in vibratory tests is 
a question which should be given attention. 

Transverse, Hardness and Bending Tests op Carbon Spring Steel 
Series A, Quenched in oil from 1500° F. 





Elastic Limit, 

(transverse) 
Lbs. per Sq. In. 


Hardness. 


Bend Test, 


Temper 

Drawn to 

Deg. F. 


Scleroscope. 


Brinell. 


Angle Bent 
through at 
Rupture, 






On Flat. 


On Edge. 


Deg. 


425 


129,137 


48.5 


47 


370 


181 


600 


136,440 


46 


50.5 


388 


60 


835 


131,017 


43.5 


49.5 


351 


86 


1025 


96,852 


34.5 


39 


268 


152 


1230 


105,400 


34 


37 


282 


167 



Series B, Quenched in oil from 1650° F. 



450 


130,922 


46 


52.5 


394 


90 


625 


134,232 


43 


57 


371 


82 


820 


141,147 


46 


56 


389 


104 


1025 


126,320 


42 


50 


371 


108 


1210 


83,457 


31 


36.5 


260 


180 



ALLOY STEEL SPRINGS 

The service conditions to which automobile springs are sub- 
jected are extremely severe, for they have to sustain the shocks at 
speed of the irregularities of the ordinary highway, built for slow- 
moving, horse-drawn vehicles. The necessity for high elastic limit, 
combined with great toughness and anti-fatigue qualities, make the 
use of alloy steel almost mandatory. 

The alloy steels in use are of the same analysis of those previously 
given under the heading of " Oil-hardened and tempered Gears " 
(q.v.). The quenching temperatures are likewise the same as 
there given, but the drawing temperatures are higher — generally 
from 850° to 1025° F. As far as static strength is concerned, the 
majority of the now common alloy compositions will give about 
the same test values, approximately : 



448 STEEL AND ITS HEAT TREATMENT 

Tensile strength, lbs. per sq. in . . . . 190,000 to 250,000 

Elastic limit, lbs. per sq. in 170,000 to 225,000 

Elongation, per cent, in 2 ins 15 to 6 

Reduction of area, per cent 45 to 20 

Some of the alloy steels, and particularly the chromium vanadium 
type, require annealing before shearing. The chromium vanadium 
steels used for springs are readily susceptible to " temper," and it 
is likely that the rapid air cooling of small flats after they leave the 
rolls will cause them to be brittle, thus giving a great amount of 
trouble in shearing. The annealing of this chromium vanadium steel 
is done by bringing the steel up to a full cherry-red heat in the 
furnace (about 1475° F.) and allowing it to cool slowly after being 
maintained at this temperature for a sufficient time to allow of uni- 
form heating. 

The new steels cannot be handled just like the old carbon steel 
springs and still obtain from them the maximum development of 
their powers. However, the new steels, being in general lower in 
carbon, will stand much abuse in heat treatment and still pro- 
duce springs of quality undreamed of a decade ago. While as a 
class spring-makers have been driven to the use of alloy steels, they 
have not as a class been forced to handle them scientifically. 

Alloy steels especially should not be heated any higher for form- 
ing than is absolutely necessaiy. Then they should always be 
reheated to the proper temperature for quenching in order to make 
sure that the entire steel is uniformly heated throughout to that 
temperature, which must be exact. The same remarks about tem- 
pering as given under carbon steel springs likewise apply here, and 
with added emphasis. 

OIL-WELL BITS 

Bits used for drilling oil wells, gas wells, etc., represent that 
class of large implements requiring "end heats." The hardening of 
these bits is necessarily an operation to be carried out in the field, 
since the bits require a more or less frequent dressing and must be 
rehardened after each heating. An extremely hard end and face, 
together with a strong, tough core and shank are the principal 
requirements for this work. 

About 6 or 8 ins. of the bit is carefully heated in the fire (usually 
a common blacksmith forge), to the proper temperature — usually 
about 1500° F. Higher temperatures should not be used unless 
absolutely required by the nature of the steel. Any scale should be 



MICELLANEOUS TREATMENTS 



449 



carefully and quickly brushed off before quenching. The bit is 
then removed from the fire and allowed to rest in a bucket of coarse 
salt for a second or two. This salt treatment may be omitted, but 
it undoubtedly gives better results; the direct use of brine is gen- 
erally too severe for most bit steels. 

A box or trough should previously be fitted with a wooden grating 
made of slats, the top of which will be about 3 or 4 ins. under the 
surface of the water in the box. Some drillers add vitriol to the 
water quenching bath to obtain a greater hardness. The bit should 
then be quickly lowered vertically into the cold water until it rests 
upon the wooden grating, and should be allowed to remain there 
until cold. 

The precautions to be observed are: (1) Lower vertically, in 
order to obtain an equal hardness on both faces of the bit; (2) do 
not quench to a greater depth than 3 or 4 ins. ; (3) do not move the 
bit nor splash the heated part of the shank with water; (4) allow 
the steel to remain in the water until cold, generally over night. 
Although the surface of the water bath may steam, it will generally 
be found that directly beneath the surface the water is cold, 
and likewise the end of the bit. Splashing the heated part of the 
bit with water has a tendency to draw the temper of the faces. 
Immersion to a greater depth than 3 or 4 ins. is apt to give a soft 
bit. If these precautions are carefully observed, and the steel is 
of the right analysis, a bit with a glass-hard surface and a strong, 
tough core will be obtained. Such bits require no tempering, and 
should not chip off. 

Oil-well bit steel will vary between 0.50 and 0.80 per cent, car- 
bon and manganese, low phosphorus and sulphur, up to 0.25 per cent, 
silicon, and the addition of about 0.5 per cent, chromium for the lower 
carbons. The chromium bit steel, if of the proper carbon-manganese- 
chromium composition, will undoubtedly give the best service. The 
following analyses are characteristic of American oil-well bits used 
and giving good service: 



Carbon. 


Manganese. 


Phosphorus. 


Sulphur. 


Silicon. 


Chromium. 


0.73 


0.61 


0.017 


0.030 


0.14 




0.59 


0.17 


0.010 


0.015 


0.13 




0.83 


0.65 


0.010 


0.021 


0.13 




0.60 


0.51 


0.012 


0.019 


0.01 


0.56 


0.54 


0.53 


0.007 


0.021 


0.006 


0.51 


0.49 


0.56 


0.010 


0.016 


0.008 


0.52 



450 STEEL AND ITS HEAT TREATMENT 

SAFE AND VAULT STEEL 

Safe and vault steel may be taken as representative of that class 
of material involving different steels welded together, but for which 
the proper treatment of one analysis will be sufficient for both. 
Steel for safes and vaults consists of alternate layers of soft and hard 
steel, and is known to the trade as " three-ply," " five-ply," etc. 
By having these alternate layers there is obtained, under suitable 
treatment, a metal which will have sufficient ductility (due to the 
soft layers) to resist explosive forces, and at the same time be im- 
penetrable to drilling, sawing or other machine operations (due to 
the "hard center"). The soft layers are made of ordinary low- 
carbon or " soft " steel, while the hard centers will analyze about 
0.85 to 1.05 per cent, carbon and manganese, with or without the 
addition of chromium. 

The plate is first machined or ground to size and the necessary 
holes drilled, threaded, and plugged with fire-clay for protection. 
The plate is then placed in a suitable heat-treatment furnace, and 
thoroughly heated to 1400° to 1500° F., depending upon the compo- 
sition of the hard layer. It is extremely important that ample 
time be allowed for the heat to penetrate and thoroughly heat 
the high-carbon steel, for it is upon the hardness of these layers 
that the full value of the finished plate will depend. The major- 
ity of the cases in which the necessary hardness was not obtained 
which the author has investigated have been due to an insufficient 
length of heating rather than to any fault in the analysis of the 
steel. 

The plate is then quickly removed from the furnace by a crane 
or hoist and quenched in cold water. As the hardness is largely 
dependent upon the rapidity with which the steel is cooled through 
the critical range, arrangements should be made to obtain a constant 
supply of cold water in contact with the steel during the quenching 
operation. If the quenching is done in a tank, the inlet supply should 
be large enough always to keep the water cold — the warm water 
being taken away from near the top of the tank. 1 In this case the 
plate is quenched vertically ; particular care should be used in getting 
the whole plate into the water as quickly as possible, and in an ab- 
solutely vertical position, if warpage is to be avoided. As soon as 
the initial immersion is accomplished the plate may be swung to 
and fro in the tank to aid in the heat removal. Other plants quench 
by means of water sprays, the plate being supported on a horizontal 



MISCELLANEOUS TREATMENTS 451 

rack; with this method of cooling the water supply should be suffi- 
cient to remove the steam as soon as it is formed. 

The plates are not tempered or drawn. Specifications require 
that the best high-speed steel drill shall not penetrate the hard- 
center layers. 

STEEL CASTINGS 

In the mad rush for alloy steels and their heat treatment but 
little attention has been given to the treatment of steel castings. 
And yet there is an opportunity for as great, if not greater, improve- 
ment in these parts as in forged or rolled sections. All steel castings 
should be annealed or oil treated, not only to remove the casting 
strains, but also to get the metal into the best possible condition. 
Due to the method of fabrication, the rapid cooling of thin sections 
and the slower cooling of adjacent thicker sections must inevitably 
produce casting strains of a more or less intense nature. Similarly 
and coincidently, the structure of the metal must inherently be poor: 
the grain will be coarse instead of fine and " silky," the metal will 
tend to have low ductility and brittleness, and the physical proper- 
ties of the steel as a whole will vary considerably. Unlike forgings 
and rolled sections, castings are not generally subjected to any 
reheating and elaboration, so that the metal must have those prop- 
erties characteristic of moderate cooling from high temperatures. 

Thus the usual specifications for steel castings, in which the low 
ductility will be apparent, will call for: 

Tensile strength, lbs. per sq. in 85,000 

Elastic limit, lbs. per sq. in 45,000 

Elongation, per cent, in 2 ins 12 

Reduction of area, per cent 18 

Even the now common addition of titanium or vanadium will not 
serve to eliminate entirely the necessity for subsequent treatment. 
Annealing, or better still, a full heat treatment, is mandatory. 

Contrary to the ideas held by many " practical " hardeners, the 
principles of treating steel castings in no wise differ from those of 
steel forgings of the same section and analysis. The main difficulty 
encountered is that caused by the length of time required for the 
diffusion of the ferrite and the equalization of the metal as a whole. 
Castings usually require considerable time for this to take place 
because of the tendency of the metal to return to its original 
molecular arrangement and structure during slow cooling. Thus 



452 STEEL AND ITS HEAT TREATMENT 

much of the unsatisfactory annealing is, technically speaking, due 
to the segregation of the ferrite. 

It is therefore necessary, in annealing steel castings, to (1) heat 
well over the upper critical range, (2) for a length of time sufficient 
to obliterate entirely the previous structure and crystallization, 
and followed by (3) slow cooling. The proper annealing temperature 
for the ordinary machinery castings will be between 1500° and 1600° 
F., depending upon the carbon content. 

If the annealing is preceded by normalizing, i.e., air cooling from 
a temperature considerably above the upper critical range — say 
1800° F. — the length of time required for the subsequent anneal 
will be considerably shortened, besides improving the steel. 

For castings with the carbon on the lower side of 0.25 or 0.30 
per cent., or for castings of considerable size, air cooling from about 
1600° F. will usually produce good results. 

The best method, however, is that of oil quenching and annealing 
or toughening — either with or without a previous normalizing. 
The castings should be heated as directed under annealing, quenched 
in the proper manner in oil, and then reheated to the temperature 
which will give the combination of strength and ductility desired. A 
drawing temperature of 1250° F. will produce the most ductile 
steel. 

STEEL WIRE 1 

The principal heat treatments used in the manufacture of wire 
are: 1, annealing; 2, patenting; 3, hardening and tempering. 

Annealing serves to accomplish three important functions: 
1. To remove the effects of hardening due to cold work in wire 
drawing or cold rolling, thus making the steel ductile and soft. 
Annealing for this purpose covers principally the low-carbon wires, 
those with carbon 0.25 per cent, and under. 2. To refine grain — 
applied principally to the higher-carbon rods and wires, those with 
carbon 0.30 per cent, and over. 3. To obtain definite structure in 
the finished material — applied principally to the higher-carbon wires, 
those with carbon 0.30 per cent, and over. 

When a steel wire rod of the structure shown in Fig. 269 is sub- 
jected to the wire-drawing process, a marked change in the grain 
structure takes place. With each successive draft, the grains stretch 
out in the direction of drafting until a point is reached when the 

1 From a paper by J. F. Tinsley, American Iron and Steel Inst., 1914, and 
The Iron Age, May 28, 1914. 



MISCELLANEOUS TREATMENTS 453 

grains have been elongated to the limit of their ductility. If sub- 
jected to further strain by further drafting they will part and the 
wire will break. Before this brittle condition is reached, therefore, 
it is necessary to heat treat the wire by subjecting it to what is 
known in the wire business as a " process annealing." 

The effect of wire drawing in elongating the structural grain 
of the steel may be seen by comparing Figs. 269, 270 and 271. Fig. 
269 shows the structure of the rod before drawing; Fig. 270 shows the 
structure after a 15 per cent, reduction from the red; and Fig. 271. 
the structure after a 60 per cent, reduction from the rod. All of 
these micrographs represent sections taken from a plane parallel 
to the axis of the rod or wire, not cross-sections. The reason for the 
marked difference in grain shown in Figs. 269 and 271 may be grasped 
more clearly when it is appreciated that Fig. 271 represents a wire 
reduced in the wire-drawing process to such a degree that it has 
become elongated 2\ times the original length of the rod. 

Process or " works " annealing consists in heating the wire to a 
certain temperature, maintaining that temperature until the entire 
mass of steel is thoroughly heated through, and finally cooling down. 
In the most common of all annealing — that to remove the effects 
of cold work such as drawing — it is not necessary to reach the 
critical temperature, which is 1300° F., or higher, depending on the 
carbon content. A temperature of 1100° F. is entirely sufficient to 
relieve the strained condition of the grain shown in Fig. 271. Fig. 
272 shows the same wire that is depicted in Fig. 271 after annealing 
at a temperature below the critical range. 

In the annealing process the strained and elongated grains 
shown in Fig. 271 break up and rearrange themselves to form a 
new grain structure as shown in the micrograph. The annealed 
steel of the structure shown is now in excellent condition to with- 
stand further cold work in reducing it to finer sizes; or, if already 
at finished size, is in good condition to meet the demands of annealed 
wire service. 

The effect of reduction of section incident to wire drawing on 
the tensile strength and ductility of steel wire, and the marked 
change brought about in these characteristics by annealing, as just 
outlined, is shown in Fig. 278. This table is based on drafting and 
annealing practice in reducing a low-carbon steel rod — in this case 
0.10 per cent, carbon — to a fine size of wire. It will be noted that 
between 80 per cent, and 90 per cent, reduction from the rod or 
annealed wire can be taken before annealing is necessary. 



454 



STEEL AND ITS HEAT TREATMENT 



It is found in practice that in cold drawing from a soft rod or 
annealed wire, the increase in tensile strength is a direct function 
of the amount of cold work, almost independent of other conditions. 




Fig. 269.— Annealed (0.08 Carbon) Steel. (Tinsley.) 

Annealing practically brings the rod or wire, regardless of size, 
back to its original condition with regard to tensile strength and 
ductility. It will be noted that the final annealing does not bring 



. 




Fig. 270.— Steel Wire (0.08 Carbon) 
Given One Draft; 15 per cent. 
Reduction from Rod. (Tinsley.) 



Fig. 271.— Steel Wire (0.08 Carbon) 
Given Several Drafts; 60 per 
cent. Reduction from Rod. 
(Tinsley.) 



the tensile strength as low as previous annealing. This is due 
simply to the fact that in annealing the fine sizes it is usual, in order 
to avoid the mechanical sticking of the wire in coils, to anneal at 
slightly lower temperatures than in ordinary process annealing. 



MISCELLANEOUS TREATMENTS 455 

The second important function of annealing is that of refining 
grain, and its practical application in the wire mill covers principally 
the medium- and higher-carbon steels. The structure of wire rods 
with regard to size of grain is dependent upon the temperature at 
which the rods are finished in the hot rolling mill and upon the rate 
of cooling through the critical temperature of the steel. In steel of 
low carbon this is not of as much importance as in the higher-carbon 
steels, for the reason that the ordinary finishing temperature varia- 
tions of good rolling-mill practice have less effect on grain structure 
of soft rods, and therefore less effect on their physical properties. 
In higher-carbon steels a fine grain is important, for it is this struc- 
ture that makes for such steels their field of usefulness, where high 
strength, high elastic limit and toughness are required. 

Theoretically, the ideal structure would be obtained if the entire 
rod could be finished at about the critical temperature. But this 
is, of course, impracticable, for the reason that it is impossible 
to regulate the finishing temperatures so closely, and for the addi- 
tional reason that there is, necessarily, particularly in rolling very 
long lengths of very small sections, a marked difference between the 
finishing temperatures of the first and last end of a rod. The higher 
the finishing temperatures above the critical range the coarser the 
grain, and the coarser the grain the more does the steel lack the 
qualities that give it value. In order to destroy the coarse or uneven 
structure that may be created as just described, it is necessary to 
anneal the steel by heating it just above its critical temperature and 
slowly cooling it down. 

The effect of overheating in coarsening the grain structure of 
a 0.45 per cent, carbon steel and the refining influence of this type 
of annealing is shown in Figs. 273 and 274. 

The third and last class of annealing to be described — that to 
obtain definite structure — is one of comparatively recent develop- 
ment in the steel-wire industry and one which promises to be of con- 
siderable value. Annealing of this type is applied principally to 
the higher carbon wires. Since the structure of such wires can be 
varied considerably within a small range of annealing temperatures, 
it covers specific products and not general classes, as would be the 
case in regard to the two previously described types of annealing, 
Figs. 275 and 276 illustrate excellently this special type of annealing. 
These photomicrographs show the structures of two annealed pieces 
of the same coil of high-carbon wire, in which the annealing temper- 
ature of the one specimen was 1300° F., and of the other 1250° F. 



456 STEEL AND ITS HEAT TREATMENT 

It is impossible to identify the structure by a simple observation of 
the fracture, which is the ordinary rough-and-ready method; nor 
is it possible to regulate annealing temperatures so closely without 
the use of pyrometers. 

In passing to the next great class of heat treatment applied to 
steel wire, patenting, it is interesting to note that we likewise pass 
to another class of wire as regards grading by carbon content. It 
naturally covers the medium-carbon steels, being employed chiefly on 
carbons between 0.35 and 0.85 per cent. In the medium-carbon 
steel wires strength and toughness are required for both process and 
finished wire. Patenting makes possible this combination of strength 
and toughness, and to this process is due in large measure a broad 
field of application for steel wire. 

The high strength and toughness of patented wire are due to 
its carbon condition and to its peculiar structure. The first step 
in the patenting process is to heat the wire to a temperature above 
its critical range. T.he degree of heating is regulated according to the 
carbon content of the steel, the size of rod or wire, and the time the 
material is subjected to the heat. After sufficient heating, the next 
step is to cool the material rapidly below its critical range, the 
structure obtained depending upon the rate of cooling. In practice, 
patenting is usually conducted as a continuous operation, the wire 
being led through the heated tubes of a furnace and cooled by being 
brought into the air or into a bath of molten lead comparatively 
cool but seldom under 700° F. 

A better understanding of the structure of a patented wire may 
be had by a comparison of the structure obtained by slow and by 
rapid cooling. If the steel after being heated is allowed to cool 
slowly through the critical temperature range, the homogeneous pre- 
existing solid solution of iron and iron carbide separates into a hetero- 
geneous mixture of two constituents, resulting in the plate-like struc- 
ture called " pearlite." In a patented wire, part of the carbide of 
iron is in solid solution and the remainder, while not in solid solution, 
has not had time to form into plates. The difference in structure 
between slow and rapid cooling is seen in Figs. 275 and 277. The 
photomicrograph of the patented wire shows, as a result of the rapid 
cooling, a structure that might be termed nondescript. Metallo- 
graphists will recognize the structure as " sorbite," which, in the 
cooling of the higher-carbon steels from above the critical tempera- 
ture, is that stage of transition just preceding the pearlitic, the final 
condition of annealed steel as shown in Fig. 275. The patented 



MISCELLANEOUS TREATMENTS 



457 






Fig. 272.— Steel Wire (0.08 Carbon) Fig. 273.— Steel (0.45 Carbon) Over 
Hard Drawn and then Annealed heated. (Tinsley.) 

below the Critical Temperature. 
(Tinsley.) 




Fig. 274.— Steel (0.45 Carbon) 
Annealed. (Tinsley.) 




Fig. 275.— Annealed (0.85 Carbon) 
Steel. (Tinsley.) 




Fig. 276.— Specially Annealed (0.85 
Carbon) Steel for Globular 
Structure. (Tinsley.) 



Fig. 277.— Patented (0.85 Carbon) 
Steel. (Tinsley.) 



458 



STEEL AND ITS HEAT TREATMENT 



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MISCELLANEOUS TREATMENTS 459 

wire, therefore, represents an unsegregated condition as against the 
segregated or coarsely laminated structure of annealed wire. The 
high tensile strength of patented wire is due to the amount of carbon 
in solution, and its toughness to the fineness of the grain structure. 

Patenting serves two important functions in the wire business: 
1. In the process of manufacture, the removal of the effects of cold 
work, such as drawing. 2. In the finished wire to give, in conjunc- 
tion with cold drawing, the required combination of strength and 
toughness. Strictly speaking, patenting is not necessary simply 
to relieve strain, for annealing would serve that purpose, but the 
structure obtained by patenting permits much further cold drawing 
than does the structure obtained by annealing. This is due primarily 
to the increased ductility and toughness of the patented wire. The 
effect of patenting as just described is shown in Fig. 279. 

In wire making, hardening and tempering should be conducted 
usually as a continuous process. In the making of tempered wire 
the material is first run through the heated tubes of a furnace, 
then quenched quickly in a bath of oil or water, then run into the 
tempering bath of, say, molten lead, each wire being in continuous 
motion from the time it enters the heating furnace until it is wound 
on a reel. Hardening and tempering apply to the higher carbon steel 
wires — those in which the carbon range is from 0.65 per cento to 
1.00 per cent. With varying tempering temperatures between 500° 
and 1100° F., the tensile strength runs from about 340,000 lbs. per 
square inch to 150,000 lbs. per square inch. At the lower temper- 
ature the decrease in tensile strength is, as we should expect, much 
greater per 100° F. range than at the higher temperatures. From 
500° to 600° F. there is a drop of 60,000 lbs. per square inch, while 
between 1000° F. and 1100° F. the drop in tensile strength amounts 
to only about 10,000 lbs. per square inch. 



CHAPTER XXI 
PYROMETERS AND CRITICAL RANGE DETERMINATIONS 

PYROMETERS 1 

Pyrometers in General. — The pyrometer has played a basic part 
in the development of intelligent heat treatment. In hardening 
rooms where pyrometers are not used, a discussion of any temper- 
ature treatment and instructions are given as the instructions 
must have been given in the Tower of Babel. There is no dis- 
tinction or mutual understanding of terms, and until a pyrometer 
— and an accurate one — is in a hardening room, it is not possible 
for those interested in the heat treatment in that room to talk 
to each other in a mutually intelligible way. Of course, where one 
old hardener has been in charge for twenty years and the manage- 
ment decides to take a chance on his staying with them and living 
for another twenty years, it may be all right to have everything 
locked up in his head; but where matters are more extensively and 
more modernly conducted, it is necessary to have some language in 
which people can talk; and the pyrometer, by virtue of its tempera- 
ture scale, which is a conventional scale of defined terms, affords 
the means of communication in a language that is mutually under- 
stood. In the same way it permits records to be kept for future 
reference. Where this is not done, men will be found trying to 
remember the heats at which they treated this, that or the other 
lot of steel; they cannot remember, and they are sure to get into 
trouble if they try to. 

There is need for a greatly extended use of pyrometers of the 
best possible grade, but more especially for an intelligent use of them 
that will in some measure compensate for the skill in producing them 
and the money involved in their installation. The pyrometer is 
not all-sufficient, nor it is the cure-all for the troubles of a hardening 
plant. There should be an education of the man to look upon 
pyrometers as gauges and indicators of the existence of energy — as 

1 It is the aim of this section to deal more with the rational use of pyrometers 
rather than with a detailed explanation of the theory and construction of the 
numerous instruments in commercial use for heat measurement. For a fuller 
explanation of the latter subject than is subsequently given, the reader is referred 
to standard reference books on the subject. 

460 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 461 

the gauge on a steam or air line, and as an aid to him in exceuting his 
work, and not as a means of releasing him from responsibility accom- 
panying the exercise of judgment. It is the use made of this energy, 
and not the mere indication of its existence, that counts. 

The pyrometer has been of inestimable value in affording a 
means to check temperature, but — and aside from the correlation 
of results — its efficiency ends largely with that indication. The 
uniformity of heated product, however, depends upon the manner 
of applying the heat, which — with the method and cost of operation 
— is primarily a function of furnace design and furnace operation. 
It is possible to indicate a uniform temperature and yet not produce a 
uniformly heated product; and unless the heat is uniformly applied 
to the stock at the temperature indicated, then a uniform pyrometer 
reading is misleading and inconclusive. Thus an elaborate pyrometer 
system, with means for signaling variations in temperature and of 
recording these variations, is not all that is necessary to control 
accurate heating. In fact such a signal system is useless unless the 
temperature is checked with the time, mass and surface of exposure. 
The pyrometer and the clock must go together, and the judgment of 
the furnace operator must go with both. 

The development toward better and cheaper results will be brought 
about by improved heating equipment and methods, even though the 
temperature recorded from any one point in a furnace chamber may 
be the same as that indicated from a similar point of another furnace 
less efficiently designed and operated. 

The time element is linked inseparably with all heating opera- 
tions. A piece of steel can absorb heat only so fast and no faster. 
Only by operating the furnace so that the maximum temperature 
is maintained for the length of time necessary uniformly to heat 
the steel throughout to that temperature, is it possible to produce 
the best results. In other words, the composition and the mass 
of the steel must be correlated with the time element. First deter- 
mine the length of time necessary, under standard furnace conditions, 
to produce the necessary results; regulate the furnace temperature, 
with the aid of the pyrometer; place a clock in view of the operator; 
watch the mass and area of surface exposed to the heat and the char- 
acter of the heat in the chamber; and then consider all these points 
together. The sooner the average heat-treatment man (and his 
superiors, for that matter) can be brought to realize that a pyrometer 
is almost valueless without the use of a clock and common-sense 
observation of furnace conditions, the better. 



462 STEEL AND ITS HEAT TREATMENT 

Thermo-Couples. — For the usual operations in heat-treatment 
work involving temperatures of over 600° or 700° F., the thermo- 
couple system is the most used. The principles upon which its use 
depends are simple. Expressed briefly, if the ends of two pieces of 
dissimilar metals (usually as wires) are joined together and one 
of the junctions (the " hot end ") is heated, the other junction (the 
" cold end ") being held at a constant temperature, a feeble electric 
current is generated in the circuit. This electromotive force, aside 
from being dependent upon the nature of the couple, is, for the 
thermo-couples in practical use, dependent upon the difference in 
temperature between the hot and cold ends. 

In regard to thermo-couples, standard base-metal compositions 
will generally give satisfaction between 600° or 700° F. and 1800° F.; 
while above 1800° F. couples of platinum and platinum-rhodium 
should be used. All base-metal couples should be readily replace- 
able, and, more emphatically, interchangeable. All couples should be 
suitably protected with iron pipes from oxidation and rough handling. 

Position of the Thermo-Couple. — The fact that a pyrometer 
may show that some particular portion of the heating zone is at the 
proper temperature is no proof that the steel is also at that temper- 
ature. The hot end of the couple may be so placed that it must 
inevitably be hotter than the hearth of the furnace, or hotter than 
any material placed on the hearth. This will be true if the end of 
the couple is exposed to the direct heat of the flame. It might 
therefore be concluded that the tip should be as near the work as 
is possible, so that both may attain the same temperature — and 
which is without doubt advisable in many instances. On the other 
hand, it has been noted in some cases in which the couples have 
been placed close to the work that the readings are not in accord 
with the temperature of the steel because the couples, being of 
smaller mass, take up readily the high peak of the flame tempera- 
ture. There are certain instances where it has been found by 
experience desirable to locate the tip of the couple in a recess in 
the furnace wall where it was out of the course of the flame and 
thus dependent for its temperature upon radiation from the main 
body of the furnace lining and radiation from the work; under 
some circumstances such a position is preferable. 

Millivoltmeter vs. Potentiometer. — By inserting into the thermo- 
couple circuit, at the cold junction, a suitable device for measuring 
the electromotive force, a reading may be obtained in millivolts; 
or, by suitable calibration, a reading directly in terms of temper- 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 463 

ature. This indicating instrument (the pyrometer) may be of the 
galvanometer or millivoltmeter type, or of the potentiometer type. 

The potentiometer in theory bears much the same relation to the 
millivoltmeter that the balance-arm scales bears to the spring scales. 
The constancy of both the spring scales and the millivoltmeter is 
entirely dependent upon the constancy of springs or of suspensions, 
and upon the absence of friction. The constancy of the potentiom- 
eter and of the balance-arm scales is dependent only upon the con- 
stancy of standard weights in one case and a standard electromotive 
force in the other. Standard weights are added to or removed 
from balance scales until a balance between known and unknown 
is obtained. Similarly in the potentiometer type varying fractions 
of a known and presumably standard electromotive force are opposed 
to the electromotive force of the thermo-couple until it is balanced 
just as a standard weight is moved along a scale arm for balance. 

This is the potentiometer not as it is, but as we would like to have 
it. The standard cell will not stay standard if any current is drawn 
from it and, consequently, the e.m.f. of the standard cell is not 
opposed to the e.m.f. of the couple in potentiometers as made for 
any ordinary use. Another cell or battery is brought into use and 
the e.m.f. of that is opposed to the e.m.f. of the couple. Now 
this secondary cell varies in e.m.f. from week to week and day to 
day, and even hour to hour under use, and it is necessary contin- 
ually to check this service cell against a standard cell and then to 
adjust for the differences that are creeping in all the time. The 
balance scales, therefore, instead of being operative with standard 
weights, have a sort of beaker of boiling water as the weight, which 
is continually boiling to less mass and which has to be filled up or 
adjusted every few minutes by comparing it with a standard weight, 
for the standard weight itself is not trusted on the scales nor is there 
any other weight, i.e., battery, that can be trusted on the scales that 
will not vary. 

Selection of Equipment.— The selection of one type or the other 
is largely a matter for economic and technical consideration. In a 
word, the purchaser should consider the relation existing between 
(1) accuracy, sensitiveness and constancy, (2) ease of reading, and 
(3) the cost — both initial and of up-keep. There is also a psycho- 
logical consideration that goes hand in hand with the above consider- 
ations and which should not be lost sight of: the millivoltmeter 
is a direct-reading instrument, which means that it is easy to read; 
the potentiometer requires a fair amount of manipulation and is 



464 



STEEL AND ITS HEAT TREATMENT 



somewhat less easy to read. The question then is: Which instru- 
ment will the average furnace man read more frequently? No 
matter how accurate a pyrometer may be, its value is only in the 
use made of it. 

Cold-end Temperature. — The cold-end temperature is a source 
of prolific error in some pyrometer installations. It should be 
remembered that all instruments are calibrated for a definite cold- 
end temperature, usually 75° F. If the cold end is in a position such 
that it receives the direct or radiating heat from the furnace, and 

Copper Lead; 



Auxiliai'y=^| 
Flexible Wire 
Couple 



Ground Line, 



Vj.-^Proti'ct'iuii 

:;:. : :;;-:':P.ipe^t- 

<V-'durietipii6f-'. 

Auxili'ilry.-B'exib'fr 

\v.n% CTuui'lc at 

const ab't'-und'ei 1 ;-. 

ground' te" m pej-af li it 




_ 



t§ Fur.nace 

■ I 



Fig. 280. — Compensating Cold End Temperatures with Auxiliary Couple. 
(Wilson-Maeulen Co.) 



therefore varies in temperature, the indicated temperature at the 
instrument will be incorrect. For this reason the cold end should 
always be kept cool, and at as near a constant temperature as is 
possible. This compensation may be accomplished i by having the 
cold end as near the ground as possible; or by letting a small stream 
of cold water flow over the cold ends; or by connecting an auxiliary 
couple of the same electromotive force as the furnace couple in oppo- 
sition to the couple in the furnace, and running the auxiliary couple 
to an underground point at the bottom of a pipe driven a few feet 
into the earth as shown in Fig. 280. The potentiometer type 
equipment frequently carries the cold end directly to the instrument, 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 465 

entirely eliminating the effect of fluctuating temperatures near the 
furnace. 

Pyrometer Standardization. — One of the most important points in 
connection with pyrometers is the necessity for frequent and regular 
calibration of the thermo-couples. All base-metal couples should 
be standardized at least once a week, and oftener if possible. Fur- 
ther, new couples should always be standardized before use, since 
errors may frequently be found even in supposedly correct new 
couples. 

There are two general methods for standardization or calibration 
of thermo-couples: (1) Checking against the melting- or freezing- 
points of known salts or metals, and (2) checking against a standard 
milli voltmeter or pyrometer. 

Standardization with Common Salt. — An easy and convenient 
method * for standardization and not necessitating the use of an 
expensive laboratory equipment is that based upon determining 
the melting-point of common table salt (sodium chloride). While 
theoretically salt that is chemically pure should be used (and indeed 
this is neither expensive noc difficult to procure), commercial accu- 
racy may be obtained by using common table-salt such as is sold by 
every grocer. The salt is melted in a clean crucible of fire-clay, iron 
or nickel, either in a furnace or over a forge-fire, and then further 
heated until a temperature of about 1600° to 1650° F. is attained. 
It is essential that this crucible be clean, because a slight admixture 
of a foreign substance might noticeably change the melting-point. 
The thermo-couple to be calibrated is then removed from its protect- 
ing tube and its hot end is immersed in the salt bath. When this 
end has reached the temperature of the bath, the crucible is removed 
from the source of heat and allowed to cool, and cooling readings are 
then taken every ten seconds on the millivoltmeter or pyrometer. 
A curve is then plotted by using time and temperature as co-ordinates, 
and the temperature of the freezing-point of salt, as indicated by 
this particular thermo-couple, is noted, i.e., at the point where the 
temperature of the bath remains temporarily constant while the 
salt is freezing. The length of time during which the temperature 
is stationary depends on the size of the bath and the rate of cooling, 
and is not a factor in the calibration. The melting-point of salt is 
1472° F. and the needed correction for the instrument under obser- 
vation can be readily applied. The curves in Figs. 281 and 282 illus- 
trate the calibration of a correct and incorrect pyrometer. 

1 Carpenter Steel Co. 



466 



STEEL AND ITS HEAT TREATMENT 























































180 
















































e 






















































100 






































A 






























































140 






































































































120 






































































































100 






































































































80 






































































































60 






































































































JO 






































































































20 






































































































n 



















































1050° 



1000' 



1550" 1500" 

Degrees Fahrenheit 



1450 c 



Fig. 281. — Diagram Showing the Calibration of a Pyrometer which Reads 45° F. 
Too High. (Carpenter Steel Co.) 



180 
100 
140 
120 



lioo 













_F 


J 


I 


f 


1 










/ i 


/ 


^-'"^ i 


--^ 


^,-^^" 





1650 



1600 



1550 1500 

Degrees Fahrenheit* 



1450° 



Fig. 282. — Diagram Showing the Calibration of a Pyrometer which is Correct. 
(Carpenter Steel Co.) 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 467 

It should not be understood from the above, however, that the 
salt-bath calibration cannot be made without platting a curve: in 
actual practice at least a hundred tests are made without platting 
any curve to one in which it is done. The observer, if awake, may 
reasonably be expected to have sufficient appreciation of the lapse 
of time definitely to observe the temperature at which the falling 
pointer of the instrument halts. The gradual dropping of the pointer 
before freezing, unless there is a large mass of salt, takes place rapidly 
enough for one to be sure that the temperature is constantly falling 
and the long period of rest during freezing is quite definite. The 
procedure of detecting the solidification point of the salt by the 
hesitation of the pointer without platting any curve is suggested 
because of its simplicity. 

Complete Calibration of Pyrometers. — For the complete calibra- 
tion of a thermo-couple of unknown electromotive force, the new 
couple may be checked against a standard instrument, placing the 
two bare couples side by side in a suitable tube and taking frequent 
readings over the range of temperatures desired. 

If only one instrument, such as a millivoltmeter, is available, 
and there is no standard couple at hand, the new couple may be 
calibrated over a wide range of temperatures by the use of the follow- 
ing standards; 

Water, Boiling-point 212° F. 

Tin, under charcoal, Freezing-point 450 

Lead, under charcoal, Freezing-point 621 

Zinc, under charcoal, Freezing-point 786 

Sulphur, Boiling-point 832 

Aluminum, under charcoal, Freezing-point 1216 

Sodium chloride, Freezing-point 1474 

Potassium sulphate, Freezing-point 1958 

A good practice is to make one pyrometer a standard; calibrate 
it frequently by the melting-point-of-salt method, and each morning 
check up every pyrometer in the works with the standard, making 
the necessary corrections to be used for the day's work. By pur- 
suing this course systematically, the improved quality of the product 
will much more than compensate for the extra work. 

Central Switch-boards. — For plants in which there are a number 
of thermo-couples, one indicating instrument with a central switch- 
board may be used. As many as sixteen couples may be wired to 
one selective switch, the maximum number simply depending upon 
the elasticity of the system and the convenience of the operator. 



468 



STEEL AND ITS HEAT TREATMENT 



A wiring diagram for such an installation is shown in Fig. 283. By 
throwing the switch from one contact to another the connection 
is made with each individual furnace. For large heat-treatment 
plants the time of one man is generally taken in attending to the 
system, he signaling the individual operators by means of lights 
and belts the relative temperatures in the furnace. We have 
previously commented upon such systems. 

The Central System. — The Chalmers Motor Company operate 
their system, 1 having two central switch-boards with sixteen furnaces 
on a switch, as follows; 




Couple 8 



Couple X 



Fig. 283. — Wiring Diagram — Pyrometer and Selective Switch. Showing Four 
Couples Connected with the Switch, Openings for Four More. 
(Hoskins Mfg. Co.) 



" We regulate the heat of the furnaces by a series of lights — 
each furnace having over it a red, blue and green light. These are 
used as follows: We will say that the temperature of an empty 
furnace which we are about to use is 1600° F. The loading of the 
furnace with forgings necessarily reduces the heat by radiation any- 
where from 100° to 250°, depending upon the number of pieces put 
in the furnace. When we commence to bring the heat up again to 
the proper place and it gets to about 1575°, the man at the switch- 
board throws on a blue light, which means to the furnace operator 
that the heat is still considerably too low. When the temperature 
reaches about 1590° the blue and green light is turned on, which 
signifies to the operator that the furnace is still not quite hot enough. 
1 Personal Correspondence. 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 469 

When the 1600° point is reached the green light is turned on; this 
is the 0. K. light and means that the temperature is correct. The 
steel is then allowed to soak at this temperature for the time neces- 
sary to affect the whole mass. If the heat during the operation 
gets too high we use signals in an inverse manner, the red and green 
lights being thrown on. If it shows a dangerous rise in temperature 
the red light is thrown on. All of these lights are accompanied by 
the ringing of a loud bell in the heat-treating department, which 
automatically attracts the attention of the man operating the 
furnaces, who at once inspect their individual furnace lights to see 
if their temperature is correct." 

DETERMINATION OF THE CRITICAL RANGES 

Critical Ranges. — The practical importance of knowing the 
exact location of the critical ranges of steel to be treated is obvious. 
Their determination by means of pyrometers is based upon the fact 
that the changes taking place in the steel at those temperatures 
involve an absorption of heating during heating (the decalescent 
points) and a giving out of heat on passing through these ranges on 
cooling (the recalescent points). 

Decalescent vs. Recalescent Points. — Before discussing methods, 
it should be stated that, for the majority of heat-treatment work, 
it is more important to know the location of the decalescent points 
than that of the recalescent points. This is for several reasons. To 
effect a complete change of the original structure of the steel, it must 
be at least heated slightly beyond the Ac3 range, regardless of the 
position of the Ar3 range. If the steel were to be heated only to the 
Ar3 range, a complete change in structure cannot take place, because 
the Ar3 range is always below the temperature of the Ac3 range. 
Further, the position of the Ar ranges is, experimentally at least, 
dependent upon the maximum temperature to which the steel is 
heated, upon the length of heating at that temperature, and upon 
the rate of cooling from that temperature. 

It should also be again stated that the determination of the 
upper critical range is of more importance than that of the lower 
critical range (Al), since the majority of hardening and annealing 
work demands a complete change of structure — which is obtained 
only above the upper critical range (Ac3). 

Temperature Difference Instruments. — American-made instru- 
ments for determining the critical ranges of steel are based either 



470 



STEEL AND ITS HEAT TREATMENT 



upon a temperature difference basis, or upon a direct record of a 
single instrument. The method used by the Leeds & Northrup 
apparatus, typical of the first class, involves the following points : 

Two bodies are heated together in the same furnace, the one 
being the steel under test and the other being a body which will 



1555 
1535 
1515 

1495 
1475 
1455 

. 1435 

.4 

ed 

K 1415 

ii> 

fl 1395 

"~-t3T& 

1355 
1335 
1315 
1295 

1275 
1255 
1235 
1215 
1195 


















1 




, 
































1 




(Bar. = .3 
Phos.= .033 
Mng. = .700 
Sin. — .253 
Sir. = .029 


























'/ 




























/ 


/ 
























1450 
Ac 


*• 

3 \ 


/ 


































\ 








































\ 


























1407 
Ac 


F. 
















































































































































136S 
A 


?' 








































































































































































































' 






































J 


' 






































































\ 























Heating.Cucve Cooling Curve 

Abscissae — Temperature Differences between Sample and Non-recalescing Body. 
Fig. 284— Transformation Curves. (Leeds & Northrup Co.) 

heat uniformly without undergoing any changes. 1 If the bodies 
are in sufficiently close contact they will heat at the same rate and, 
barring changes in one which do not occur in the other, will remain 
equal in temperature. When, however, the steel undergoes an inter- 
nal change involving absorption or liberation of heat, its temperature 
changes relatively to the other body and a temperature difference is 
set up between the two. Hence the apparatus for the location of 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 471 

critical points by this method is designed to do two things: first, 
to measure the temperature of the sample; second, to indicate the 
temperature relationship between the sample and the unchanging 
body. A curve using temperatures as ordinates and temperature 
differences as abscissae is the best way of making use of the results. 

Temperature Difference Records.— Fig. 284 is a reproduction of 
such a plot. From the start of the test until 1205° the temperature 
difference is small and constant. When the temperature of 1369° 
is reached a sudden increase in the temperature difference takes 
place, the Ael range. As soon as this sudden change ceases (i.e., 
transformation is completed), the sample and the unknown begin 




Fig. 285.— Critical Range Curve on a Direct-reading Apparatus. Carbon 
0.44 per cent.; Manganese, 0.53 per cent.; Phosphorus, 0.035 per 
cent.; Sulphur, 0.025 per cent.; Silicon, 0.028 per cent. 

to equalize in temperature and the record of their decreasing differ- 
ence follows a typical cooling curve between 1380° and 1455°, except 
at about 1407°, where the Ac2 transformation begins to affect the 
record. At 1407-1410° this Ac2 change is completed. Again at 
1450° there is a departure from a smooth curve; this is the beginning 
of the third transformation, which transformation is not completed 
until about 1495°. This is the Ac3 transformation. On cooling, 
the reverse takes place, except that the two upper points occur 
closer together and appear as one. The lowest range is clear cut. 

Direct-reading Instruments.— Fig. 285 shows a record obtained 
from a Bristol instrument. Leaving aside a discussion of the 



472 STEEL AND ITS HEAT TREATMENT 

scientific pros and cons, the three main objections to this class of 
instrument are: (1) the small area covered by the record, involving 
less accuracy; (2) lack of that degree of sensitiveness which is 
necessary to bring out the upper critical ranges; and (3) a curve 
showing direct temperatures instead of temperature difference. 

Practical Method for Determining Critical Ranges. — For plants 
which have to determine the critical ranges but infrequently less 
costly apparatus may be used. The outfit should consist of a thermo- 
couple made of small wires so as to respond quickly to any slight 
variation in temperature; the necessary leads; and a sensitive 
millivoltmeter or pyrometer with a finely divided scale. This 
instrument may also be used as a standard, or checking instrument, 
for calibration work. The specimens to be tested should be small 
so as to heat uniformly and quickly. These may be either a small 
cylinder, say f in. diameter by 1| in. long, or duplicate pieces each 
1^ in. long by f in. wide by \ in. thick. In the former case the end 
of the couple is inserted in a small hole drilled through the axis of 
the cylinder to a depth of about \ in.; in the latter case the pieces 
are clamped together, one on either side of the end of the thermo- 
couple so as to form a tight contact. The specimen is then heated 
in any convenient manner, readings being taken every few seconds 
as the critical ranges are reached. When the indicated temperature 
is well above the upper critical range, the specimen is removed from 
the heat, allowed to cool not too rapidly, and readings taken to 
obtain the Ar ranges. The temperature readings, or difference in 
readings, should then be plotted against the time to obtain the 
necessary curves. 



PYROMETERS AND CRITICAL RANGE DETERMINATIONS 473 



TEMPERATURE CONVERSION TABLE 
By Dr. Leonard Waldo 

Reprint from Metallurgical and Chemical Engineering. 



c. ° 





10 


20 


30 


40 


50 


60 


70 


80 


90 






-200 
-100 
- 


F. ° 
-328 
-148 
|- 32 


F. ° 
-346 
-166 
+ 14 


F. ° 
-364 
-184 
- 4 


F. ° 

-382 
-202 
- 22 


F. ° 
-400 
-220 
- 40 


F. ° 
-418 
-238 
- 58 


F. ° 
-436 
-256 
- 76 


F. ° 
-454 
-274 
- 94 


F. ° 

-292 
-112 


F. ° 

-310 
-130 







32 


50 


68 


86 


104 


122 


140 


158 


176 


194 






100 


212 
392 
572 

752 

932 

1112 

1292 
1472 
1652 


230 
410 
590 

770 

950 

1130 

1310 
1490 
1670 


248 
428 
608 

788 

968 

1148 

1328 
1508 
1688 


266 
446 
626 

806 

986 

1166 

1346 
1526 
1706 


284 
464 
644 

824 
1004 
1184 

1364 
1544 
1724 


302 
482 
662 

842 
1022 
1202 

1382 
1562 
1742 


320 
500 
680 

860 
1040 
1220 

1400 
1580 
1760 


338 
518 
698 

878 
1058 
1238 

1418 
1598 
1778 


356 
536 
716 

892 
1076 
1256 

1436 
1616 
1796 


374 
554 
734 

914 
1094 
1274 

1454 
1634 
1814 


C. ° F. ° 


200 
300 

400 
500 
600 

700 

800 
900 


1 
2 
3 

4 
5 
6 

7 
8 
9 

10 


1.8 
3.6 
5.4 

7.2 

9.0 

10.8 

12.6 
14.4 


1000 


1832 


1850 


1868 


1886 


1904 


1922 


1940 


1958 


1976 


1994 


16.2 


1100 
1200 
1300 


2012 
2192 
2372 

2552 
2732 
2912 

3092 
3272 
3452 


2030 
2210 
2390 

2570 
2750 
2930 

3110 
3290 
3470 


2048 
2228 
2408 

2588 
2768 
2948 

3128 
3308 

3488 


2066 
2246 
2426 

2606 
2786 
2966 

3146 
3326 
3506 


2084 
2264 
2444 

2624 
2804 
2984 

3164 
3344 
3524 


2102 
2282 
2462 

2642 
2822 
3002 

3182 
3362 
3542 


2120 
2300 
2480 

2660 
2840 
3020 

3200 
3380 
3560 


2138 
2318 
2498 

2678 
2858 
3038 

3218 
3398 
3578 


2156 
2336 
2516 

2696 
2876 
3056 

3236 
3416 
3596 


2174 
2354 
2534 

2714 
2894 
3074 

3254 
3234 
3614 


18.0 


1400 
1500 
1600 




F. ° 


C. ° 


1700 
1800 
1900 


1 
2 
3 

4 
5 
6 

7 
8 
9 

10 
11 
12 

13 
14 
15 

16 

17 

18 


.56 
1.11 
1.67 


2000 


3632 


3650 


3668 


3686 


3704 


3722 


3740 


3758 


3776 


3794 


2.22 


2100 
2200 
2300 

2400 
2500 
2600 

2700 
2800 
2900 


3812 
3992 

4172 

4352 
4532 
4712 

4892 
5072 
5252 


3830 
4010 
4190 

4370 
4550 
4730 

4910 
5090 
5270 


3848 
4028 
4208 

4388 
4568 

4748 

4928 
5108 

5288 


3866 
4046 
4226 

4406 
4586 
4766 

4946 
5126 
5306 


3884 
4064 
4244 

4424 
4604 
4784 

4964 
5144 
5324 


3902 
4082 
4262 

4442 
4622 
4802 

4982 
5162 
5342 


3920 
4100 

4280 

4460 
4640 
4820 

5000 
5180 
5360 


3938 
4118 
4298 

4478 
4658 
4838 

5018 
5198 
5378 


3956 
4136 
4316 

4496 
4676 
4856 

5036 
5216 
5396 


3974 
4154 
4334 

4514 
4694 
4874 

5054 
5234 
5414 


2.78 
3.33 

3.89 
4.44 
5.00 

5.56 
6.11 
6.67 

7.22 


3000 


5432 


5450 


5468 


5486 


5504 


5522 


5540 


5558 


5576 


5594 


7.78 
8.33 


3100 
3200 
3300 

3400 
3500 


5612 
5792 
5972 

6152 
6332 
6512 

6692 
6872 
7052 


5630 
5810 
5990 

6170 
6350 
6530 

6710 
6890 
7070 


5648 
5828 
6008 

6188 
6368 
6548 

6728 
6908 
7088 


5666 
5846 
6026 

6206 
6386 
6566 

6746 
6926 
7106 


5684 
5864 
6044 

6224 
6404 
6584 

6764 
6944 
7124 


5702 
5882 
6062 

6242 
6422 
6602 

6782 
6962 
7142 


5720 
5900 
6080 

6260 
6440 
6620 

6800 
6980 
7160 


5738 
5918 
6098 

6278 
6458 
6638 

6818 
6998 
7178 


5756 
5936 
6116 

6296 
6476 
6656 

6836 
701-6 
7196 


5774 
5954 
6134 

6314 
6494 
6674 

6854 
7034 
7214 


8.89 
9.44 

10.00 


3600 

3700 
3800 
39C0 




C. ° 





10 


20 


30 


40 


50 


60 


70 


80 


90 







Examplfs: 1347° C. 2444° F. +12°.6 F. = 2456°.6 F.: 3367° F. = 1850° C. +2°.78 C.= 
1852°.78 C. 



INDEX 



Abrasion, resistance to, 14 

Abrasive wear in Mn steels, 391 

Air Control, 25 

Air cooling, 131, 132, 289, 313 

Air hardening, 286, 289 

Allotropic ferrite, 106 

Alloy steel. See Chapters XIII, et seq. 

necessity for heat treatment, 1, 295 
Alpha ferrite, 106 
Alternating impact tests, 10 
American gas furnace process, 245 
Animal charcoal. See Charcoal. 
Annealing. See Ch. VII and Alloy 
Steels. 

air cooling after, 132, 133 

at critical range, 118 

beyond critical range, 119 

behavior of excess ferrite in, 126 

box, 154 

commercial, 116, 156, 288 

definition of, 116 

double, 148 

effect of, 116, 130 

effect of initial structure in, 133 

elemental considerations in, 117 

for maximum ductility, 151 

for strength-ductility, 151 

full, 116 

furnace cooling after, 127 

hyper-eutectoid steels, 156, 290 

hypo-eutectoid steels, 118 

length of, 126 

maximum overheated steel, 146 

overheated steel, 141 

pit, 132 

rate of cooling after, 127, 132 

rate of heating in, 123 



Annealing: 

severely overheated steel, 143 

size of object, 156 

slow cooling after, 132 

special methods, 135, 148, 151 

structures from, 126 

temperature for, 117, 122, 155, 271, 
274, 278, 287, 338 

time experiments, 124 

tool steel, 153 

vs. toughening, 20^ 

wire, 452 
Armor plate, 362 
Arrangement of charge, 55 
Ar ranges, 109 

Atmosphere in furnace, 28, 29, 152 
Austenite, 104, 161, 271, 393 
Automatic furnaces: 

for die blocks, 427 
Automobile steel, 1, 274, 372 
Axles, 1, 8, 9, 185, 276, 281, 283, 285, 
287, 358, 372, 383 



B 



Ball-bearings, 336 

Ballistic tests, 14 

Barium carbonate, 225, 231, 235 

Baths : 

heating, 170 

salt, 171 

tempering, 194 
Best case, 259, 261 
Beta ferrite, 108 
Bit steel, 448 
Bolts, carburizing of, 239 
Boring, hollow, 183 
Box annealing, 154 
Boxes for carburizing, 238 



475 



476 



INDEX 



Brains, purchasing of, 77 

Brine, 175 

Brinell hardness. See Hardness. 

of carbon steels, 12, 269 

of chromium-nickel steels, 361, 363 

of chromium-vanadium steels, 380 

of nickel steels, 327 
Brittleness, 3, 7, 8, 9, 207, 273, 280, 286, 
305, 307, 308, 312, 349, 387, 398 

Stead's, 155 
B.T.U. values, 18, 24 
Burners, 25 



C 



Calcium chloride for quenching, 176 
Calibration of pyrometers, 467 
Capacity of the steel, 204 
Carbides, 333, 347, 378, 393 
Carbon : 

concentration of, 220, 308 

direct action in carburization, 212 

maximum in case, 261, 313 

plus carbon monoxide, 222 

solution of in carburization, 223 
Carbon content: 

for tools, 412 

influence of, 3, 207 

in manganese steels, 391 

in nickel steels, 301 
Carbon monoxide, 213, 215, 220 
Carbon steel: 

under 0.15 per cent., 270 

0.15-0.25 per cent., 272 

0.25-0.35 per cent., 276 

0.35-0.40 per cent,, 280 

0.45-0.60 per cent., 286 

over 0.60 per cent., 289 
Carbonates, 214 
Car furnaces, 61 
Carburization: 

boxes, 238 

carbon monoxide plus hydrocar- 
bons, 220 

carbon plus carbon monoxide, 222 

depth of penetration, 224 

effect of chromium, 333 

gas process, 245 

heat treatment requirements, 251 



Carburization: 

object of, 210 

of chromium-nickel steels, 351 

of nickel steels, 304, 307, 3Q8, 310 

requirements of, 210 

steel for, 211, 272 

temperature of, 223, 230, 231, 232, 
252 

with carbon monoxide, 213, 215 

with simple solid cements, 233 

wood charcoal, 213 
Case carburizing. See Carburization. 
Case hardening: 

gears, 440-443 

maximum efficiency in, 261 

treatment of hyper-eutectoid steels, 
253 

treatment of hypo-eutectoid steels, 
252 
Case, the best, 259, 261 
Castings, 135, 148, 155, 156, 451 
Cementine, 93, 101, 212, 253, 259, 261, 

265, 290, 334, 378, 388, 445, 456 
Centigrade tables, 473 
Central pyrometer systems, 467 
Chamber, height of, 42 

twin-furnaces, 68 
Changes : 

in diameter, 422 

in heating, 109 

in length, 422 
Charcoal, 231, 233, 234 
Charge: 

height of, 42 

influence in heating, 47 

influence of arrangement, 47, 56 
Charging, 117 

Chemical composition, effect of, 1 
Chipping chisels, 420 
Chisels, 335, 420 
Chromium : 

influence in carburization, 333, 334 

in high speed steel, 406 

in manganese steels, 393 

vs. silico-manganese, 397 
Chromium steels: 

general characteristics, 332 

0.5 chromium, low carbon, 334 

0.5 chromium, 0.35-0.50 carbon, 335 



INDEX 



477 



Chromium steels: 

0.5 chromium, over 0.50 carbon, 335 

1.5 chromium, 336 

2.0 chromium, 344 

high chromium, 346 
Chromium-nickel steels: 

carburization, 352 

gears, 444 

heat treatment, 352 

low Cr, low Ni, 353 

0.5 Cr, 2.5 Ni, 365 

0.6 Cr, 3.5 Ni, 364 

0.75 Cr, 3.0 Ni, 365 

1.0 Cr, 1.75 Ni, 365 

1.5 Cr, 3.5 Ni, 362 

Mayari, 372 

special analyses, 370 

vs. chromium- vanadium, 349, 378 
Chromium- vanadium steels. See Vana- 
dium. 
Circulation for cooling oil, 179 
Classification of: 

gear steel, 440 

heat treatment after carburization, 
252 

nickel steel, 295 
Coal furnaces, 59 
Coalesced structure, 127 
Coffin process, 284 
Cold crystallization, 271 
Cold-end temperatures, 462, 464 
Cold roUing, 115 

vs. strength, 6 
Cold work, effect on structure, 115 
Color chart, 423 
Colors in tempering, 192 
Combustible mixture, 20 
Combustion, furnace atmospheres 

from, 29 
Commercial annealing, 116, 156, 289 
Commercial data in carburization, 234 
Commercial ratio of chrome and nickel, 

350 
Compensation of pyrometers, 464 
Compressed air in quenching, 180 
Compressive strength, 5 
Conservation of heat, 64 
Continuous furnaces, 69 
Contraction in hardening, 183 



Contraction of area, 5 

Conversion, temperature, 473 

Cooling, after annealing, 126, 271, 338 

Cooling the oil bath, 177 

Cooling the water bath, 177 

Cost of heating, 16, 17, 31 

Couples, 462 

Cracking, 182, 286 

Crank shafts, 1, 372 

Critical ranges, 102, 469 

changes at, 117 

effect of manganese, 387 

effect of nickel, 295, 302 

heating over the, 118 

merging of, 108 

of chromium steel, 332, 337, 345 

of chromium-nickel steel, 252, 370 
Critical ranges of high-carbon steel, 
290 

of high-speed steel, 407 

of hyper-eutectoid steel, 156 

of manganese steel, 391, 395 

of tool steel, 417 
Cutters, 432 
Cyanide hardening, 246 
Cyanides in carburization, 215, 237 



D 



Dead soft steel, 270 
Decalescence, 469 
Deck plate, 352, 362 
Depth of penetration, 309 
Design of furnace, 16 
Determination of critical ranges, 469 
Diameter, effect on tests, 269 
changes in, 422 
. Die blocks, 124, 335, 423 
Dies, 287, 436 
Differential hardening, 177 
Diffusion, 135, 278, 304 
Distortion, 317 
Distribution of carbon, 224 
Door heights, 41 
Double annealing, 148 
Double carbide steel, 332 
Double quenching, 189, 259, 277, 289, 

313 
Double regenerative quenching, 262 



INDEX 



Drawing of wire, 452 
Drilling hollow, 183 
Drills, 335, 430 
Drop tests, 8 

Ductility, 5, 7, 151, 203, 294, 349 
Duplex process, 372 
Duplication of results, 2, 204 
Dynamo sheet iron, 398 
Dynamic strength, 2, 207, 276, 349, 
350, 362, 380 



E 



Effect of: 

chromium, 332, 406 

of initial structure, 133 

manganese, 387 

mass, 323, 365, 373, 417 

nickel, 294, 349 

silicon, 396 

vanadium, 378 
Elastic limit, 3, 5, 119 
Electricity : 

atmospheres with, 30 

for heating, 26 
Electromagnets, 398 
Elongation, 5, 119 
Endurance, 6. 10 
Enfoliation, 218, 308, 316 
Engine forgings, 272, 274 
Engraved, dies, 428 
Equalization, 135, 304, 306 
Equalizing action of carbon monoxide, 

222 
Equipment, pyrometer, 463, 
Eutectoid for nickel steel, 304 

steel, 94 
Expansion in hardening, 183 
Excess ferrite, behavior on cooling, 126 



F 



Fahrenheit tables, 473 

Failures of heat-treated axles, 286 

Fatigue, 2, 6, 9, 276, 350 

Ferrite, 100, 106, 118, 127, 294, 295 

Ferro-cyanides, 225, 237 

Files, 335, 346, 433 

Finishing temperature for forging, 85 



Fire-ends, 462 

Five-ply steel, 450 

Flanges, 272 

Floor space, 24 

Flue construction, 64, 70 

Force, 2 

Forging. See Chapter V. 

Forging temperatures for tool steel, 

417 
Fragility, 9 
Frames, automobile, 1 
Fuel: (See Chapter II) 

cost of dehvering, 24 

costs, 18, 26 

distribution, 22 

efficiency, 20 

equipment, 24 

fluid, the, 21 

oil, 25 

oil, air control with, 76 

producer gas, 23 

question, 31 

selection of, 22, 31 

the right, 20 
Fuel: 

vs. furnace design, 26 

vs. operations, 21 j 

vs. product, 20 
Furnace : 

atmospheres, 28, 29 

batteries, 68 

cooling in toughening, 205 

design, 16, 26, 50, 69, 89 

equipment, 31, 68, 69 

guarantees, 36 

plans, 64 

temperature of in heating, 123 

the one, 36 
Furnaces : 

car-bottom, 61 

coal, 59 

continuous, 427 

forge, 89 

general considerations, 68 

muffle, 48 

overtired, 54 

perforated arch, 53 

practical notes on, 69 

rotary, 51 



INDEX 



479 



Furnaces: 

semi-muffle, 50 
twin-chamber, 68 
underfired, 37, 62 
unit system, 67 

G 

Gamma ferrite, 109 

Gas. See Fuel. 

Gases, action of, in carburization, 213 

Gears, 1, 239, 276, 287, 372, 397, 440 

Grade, in tool steel, 411 

Gradual cements, 237 

Grain size : 

at Ac3, 118 

beyond Ac3, 110, 119 

effect of work on, 115 

in carburization, 232 
Gun barrels, 203, 275, 389, 399 

forgings, 186, 281, 400 

H 

Hardening: 

changes on, 159 

cyanide, 246 

definition, of, 159 

differential, 177 

heating for, 165 

high speed steel, 410 

nickel steels, 304 

pack, 246, 249 

relation to annealing, 160 

strains, 192 

superficial, 246 

temperature for, 164, 272, 277, 281, 
289, 290, 311, 323, 379, 424 

tool steel, 416 
Hardness, 11, 270, 309, 327, 333, 338, 
361, 363, 380, 414 

Brinell 11. Also see Hardness. 

cutting, 290 

due to chromium, 333 

scleroscope, 13, 266. Also see 
Hardness. 

wearing, 290 
Heat, quality of, 28 
Heat application. See Chapter III. 



Heat conservation, 18, 64 
Heat reservoir, 41 
Heat treatment: 

definition of, 103 

growth of, 1 

necessity for, 1 
Heating : 

changes on, 109, 118, 159 

costs, 16, 17 

distinctive conditions in, 16 

factors, 17 

for carburization, 240 

for forging, 82, 84 

for hardening, 165 

for tools, 421 

in lead, 433 

in salt, 171, 264 

influence of chromium in, 332 

large sections, 124 

length of, 126, 305, 424 

problem, 69 

prolonged, 126, 349 

rate of, 123 

uniform, 32 

unit, standard, 17 
Heating : 

with electricity, 26 
Height of chamber, 42 
Height of charge, 42 
Height of door, 41 
High-carbon case, treatment of, 257 
High speed steel. (See Chapter XVIII). 

chemical composition, 403 

chromium in, 406 

critical ranges of, 407 

hardening of, 410 

Taylor- White method, 408 

Tungsten in, 404 
High temperature carburization, 232 
High temperatures, effect on springs, 

445 
Hollow boring, 183 
Hollow tools, 437 
Hot-ends, 462 
Hot work, effect of, 115 
Human element. (See Chapter IV.) 

effect on improvement, 75 

effect on operation, 76 

importance of, 73, 116 



480 



INDEX 



Human element: 

in forging, 90 

vs. basic heat treatment conditions, 
74 

vs. pyrometers, 460, 461 
Hydro-carbons, 216, 220 
Hyper-eutectoid steel, 94 

annealing of, 156 

zones in carburization, 218 
Hypo-eutectoid steel, 94 

annealing of, 118 



Impact strength, 2, 8, 9, 203, 232, 266 
Impurities in carburization, 212 
Influence. See Effect. 
Initial heating for forging, 82 
Initial heating for annealing, 117 
Initial structure, effect of, 133 
Intensifiers, 380 
Intermediary types of carburized zones, 

220 
Interrupted regenerative quenching, 

264 



Jar steel, 335 



Knives, 335 



K 



Labor, necessity for skilled, 77 

selection of, 79 
Laminations, 294 
Lead baths, 170, 197, 433 
Ledges, 38 
Length, change in, 420 

of heating, 126, 424 
Levers, 272 
Liquation, 227, 309 
Locomotive axles, 283 
Low-temperature carburization, 231 



M 



Machine parts, 280 
Machinery, steel, 272 



Machining: 

effect on structure, 115. 

quality, 294 
Magnet steel, 398 
Magnet, use in hardening, 167 
Manganese : 

in carburization, 211 

on hardening, 190 

on machining, 271 

steels, 387 
Martensite, 162 
Martensitic steels, 295, 346 
Mass, influence of, 32, 43, 223, 224, 269, 

365, 384, 417 
Mayari steel, 372 
Mechanical mixture, 93 
Mechanical work, effect in annealing, 

136, 138 
Methods for annealing, 155 
Microstructure : 

of high-carbon steels, 290 

of nickel steels, 296 
Milky-ways, 137 
Milling cutters, 432 
Milhvoltmeters, 462 
Mineral hardness, 333 
Mixed cements, 309 
Molybdenum steels, 399 
Motion in hardening, 169 
Muffle furnaces, 48 



N 



Natural alloy, 372 

Natural, steel in the, 1, 3 

Navy specifications for tool steel, 414 

Network, 111, 126, 127 

Nickel : . 

effect on physical properties, 302 

influence of, 304 

influence on critical ranges, 302 
Nickel steels: 

2 per cent., 301, 311 

3.5 per cent., 311, 313, 353, 358 

5 per cent., 301, 304, 305,313,317,328 

10 per cent., 301 

25-35 per cent., 301, 328 

carburization of, 307 

for gears, 444 



INDEX 



481 



Nickel-chromium steel. See Chromium 

nickel. 
Nickel-vanadium steel, 385 
Nitrogen in carburization, 214 
Normalizing, 157 
Nuts, 272 

O 

Obstructing agents, 161, 295 

Oil. Also see Fuel Oil. 
quenching speed of, 173 
vs. water for hardening, 283 

Oil baths, 177, 196 

Oil burners. See Burners. 

Oil tempering, 175 

Oil-tempered gears, 440, 443 

Oil-well bits, 335, 448 

Operation of forge furnaces, 89 

Operators, value of, 79 

Oscillating temperatures, 230 

Osmondite, 161 

Overtired furnaces, 54 

Overheated steel, 82, 141 

Oxidation, protection from, 153, 425 

Oxygen, action in carburization, 213 



Pack hardening, 246, 249 

Packing for carburization, 239 

Patenting, 456 

Pearlite, 93, 100, 104, 152, 207, 295, 302 

effect of cooling on, 152 
Penetration, depth of, 224, 309 

velocity of with chromium, 334 
Perforated arch furnaces, 53 
Phosphorus, 3 

Physical properties at Ac3, 118 
Pins, 272 
Pit annealing, 132 
Polyhedral steels, 296 
Potentiometers, 462 
Preheating, 117, 424 
Process annealing, 453 
Producer gas, 23 

Prolonged heating of nickel steels, 308 
Propeller shafts, 371, 399 
Protection of steel, 153, 425 
Protective deck plate, 352, 362, 367 



Punches, 436 

Punching, effect on structure, 115 
Purchasing brains, 77 
Pyrometers, 460 
standardization, 465 

Q 

Quality of heat, 28 

Quality of product vs. first cost, 16 

Quench-toughening, 208 

Quenching: 

after tempering, 194 

baths, 172 

best temperatures, 167 

double, 189 

manner of, 185 

media, 172, 206 

special methods, 175 

speed, 172 

tanks, 182 

water for, 175 

R 

Radiation systems for cooling oil, 179 
Rate of cooling, 126, 132, 289 
Rate of heating, 123 
Razors, 335 
Reamers, 436 
Recalescence, 469 
Reduction of area, 5, 119 
Refinement, 110, 141, 143, 146, 160, 

257, 271, 277, 278, 288, 455 
Regeneration, 23, 256, 257, 311, 317 
Relation of austenite to carbide, 393 
Relation of physical tests, 10 
Requirements of gears, 440 
Resilience, 8 

Rifle barrels; 203, 389, 399 
Rings, 437 
Rivet sets, 438 
Roller bearings, 336 
Rotary bending, 6 
Rounds, hardening of, 186, 188 

S 

Safe steel, 450 

Salt, use in carburizing, 235 

standardization of pyrometers, 465 



482 



INDEX 



Salt baths, 171, 197, 264 

Sand baths, 196 

Saturation, 5, 32, 203, 389 

Saws, 335, 439 

Scleroscope, 13, 228, 327, 361, 380 

Screw stock, 280 

Screws, carburizing, of 239 

Seams, 294 

Selection of men, 79 

Selection of pyrometer equipment, 465 

Selection of tool steel, 411 

Sensitiveness of manganese steels, 390 

Sensitiveness of silico-manganese steels, 

396 
Shafts, 185, 327 
Shock, resistance to, 312 
Shore-hardness. See Scleroscope. 
Silicon steels, 396 
Silicon-manganese steels, 396, 444 
Size of section, 123, 124, 269, 288 
Skilled labor, 77 
Slow cooling, 110, 127, 132, 205, 254, 

263 
Soft forging steel, 276 
Solid solutions, 104 

Solution of carbon in carburization, 223 
Sorbite, 152, 163, 198, 207 
Specifications for tool steel, 414 
Spheroidal cementite, 157, 260, 343, 

456 
Springs, 1, 445 

Static strength, 1, 2, 101, 362 
Standard heating unit, 17 
Standardization of results, 12, 207 
Stead's brittleness, 155 
Steam hardening, 286 
Steering parts, 1 
Strength-ductility, 151 
Stresses and strains, 2, 6, 115, 192, 205 
Structure, definition of, 102 
Structure of slowly cooled steel, 94, 126 

effect of annealing on, 133 
Structure from air cooling, 132 
Sudden cements, 231, 237 
Suddenly applied loads, 7 
Sulphur diffusion, 241 
Summary for case hardening, 265 

for annealing, 155, 156 
Superficial hardening, 246 



Table for temperature conversion, 473 

Tank, size of quenching, 182 

Taps, 420 

Taylor-White method, 408 

Temper, 412 

colors, 192, 193 
Temperature : 

conversion table, 473 

effect on grain size, 110 

effect on network, 111 

variation, 32 
Temperature of: 

annealing, 116, 122 

carburization, 226, 230, 252 

hardening, 164 

pack hardening, 249 

quenching bath, 172 

toughening, 201 
Tempered axles, 284 
Tempering: 

color for tools, 415 

definition, of, 191 

for depth, 193 

gears, 443 

handling material in, 196 

methods, 195 

oil, 175 

plate, 195 

quenching after, 194 

springs, 446 
Tensile strength, 2 

of cementite, 101 

of ferrite, 100 

of pearlite, 100 
Testing: 

comparative results, 269 

errors in, 350 

purpose of, 1 
Tests from center, 269 
Thermo-couples, 462 
Threading, treatment for, 271 
Tie rods, 272 

Time of heating, 271, 462 . 
Tool steel, annealing of, 152 

proper carbon for, 412 

selection of, 411 
Torsional strength, 1, 5 



INDEX 



483 



Tough-hardness, 333 
Toughening, 198 

high vs. low temperature, 206 

range, 200 

temperature vs. mass, 373 

vs. annealing, 207 

vs. ductility, 203 

vs. impact strength, 203 
Toughness, 271, 414 
Transference numbers, 12, 270, 327, 

361, 364, 380 
Transition constituents, 152 
Troostite, 162, 191 
Tungsten steel, 398, 403 
Twin chamber furnaces, 68 



U 



Underfired furnaces, 37, 62 

Underfiring, 37 

Uniform heating, 37 

Uniformly heated product, 32 

Unit surface system, 67 

Uses of chromium nickel steel, 372 



Value of furnace operator, 79 
Valve stems, 272 
Vanadium, effect of, 378 
Vanadium steels: 
gears, 444 



Vanadium steels: 

nickel, 385 

Type A, 382 

Type D, 383 

Type G, 384 
Vault steel, 450 
Velocity of penetration with chromium, 

334 
Vents, 39, 89 
Vibratory stresses, 1 



W 



Warping, 184, 272 

Water bath, cooling the, 177 

Water quenching, 175, 286 

Water spray, 174 

Water toughening, 395 

Water vs. oil for hardening, 283 

Wear, 1, 14, 280, 333, 391 

Welding of alloy steel, 372 

Welding properties of tool steel, 417 

Well bits, 385, 448 

Wire, 452 

Wood charcoal, 213, 225 

Work, effect on grain size, 115 

Working strength, 4 

Works, annealing, 453 



Yield point, 4 



